diff --git a/src/FsCodec.NewtonsoftJson/UnionConverter.fs b/src/FsCodec.NewtonsoftJson/UnionConverter.fs
index c5c63820..0472a484 100755
--- a/src/FsCodec.NewtonsoftJson/UnionConverter.fs
+++ b/src/FsCodec.NewtonsoftJson/UnionConverter.fs
@@ -20,7 +20,7 @@ module private Union =
     let isUnion = memoize (fun t -> FSharpType.IsUnion(t, true))
     let getUnionCases = memoize (fun t -> FSharpType.GetUnionCases(t, true))
 
-    let createUnion t =
+    let private createUnion t =
         let cases = getUnionCases t
         {
             cases = cases
diff --git a/src/FsCodec.SystemTextJson/FsCodec.SystemTextJson.fsproj b/src/FsCodec.SystemTextJson/FsCodec.SystemTextJson.fsproj
index fd22c065..8056225b 100644
--- a/src/FsCodec.SystemTextJson/FsCodec.SystemTextJson.fsproj
+++ b/src/FsCodec.SystemTextJson/FsCodec.SystemTextJson.fsproj
@@ -26,7 +26,8 @@
 
     <PackageReference Include="FSharp.Core" Version="4.3.4" />
 
-    <PackageReference Include="System.Text.Json" Version="5.0.0-preview.3.20214.6" />
+    <PackageReference Include="System.Text.Encodings.Web" Version="5.0.0" />
+    <PackageReference Include="System.Text.Json" Version="5.0.0" />
     <PackageReference Include="TypeShape" Version="8.0.0" />
   </ItemGroup>
 
diff --git a/src/FsCodec.SystemTextJson/UnionConverter.fs b/src/FsCodec.SystemTextJson/UnionConverter.fs
index 9a6c0b3a..fec3df35 100755
--- a/src/FsCodec.SystemTextJson/UnionConverter.fs
+++ b/src/FsCodec.SystemTextJson/UnionConverter.fs
@@ -65,13 +65,14 @@ module private Union =
     /// Parallels F# behavior wrt how it generates a DU's underlying .NET Type
     let inline isInlinedIntoUnionItem (t : Type) =
         t = typeof<string>
-        //|| t.IsValueType
+        || (t.IsValueType && t <> typeof<JsonElement>)
         || t.IsArray
         || (t.IsGenericType
            && (typedefof<Option<_>> = t.GetGenericTypeDefinition()
                 || t.GetGenericTypeDefinition().IsValueType)) // Nullable<T>
 
-    let typeHasJsonConverterAttribute = memoize (fun (t : Type) -> t.IsDefined(typeof<Serialization.JsonConverterAttribute>, false))
+    let typeHasJsonConverterAttribute = memoize (fun (t : Type) -> t.IsDefined(typeof<Serialization.JsonConverterAttribute>(*, false*)))
+    let typeIsUnionWithConverterAttribute = memoize (fun (t : Type) -> isUnion t && typeHasJsonConverterAttribute t)
 
     let propTypeRequiresConstruction (propertyType : Type) =
         not (isInlinedIntoUnionItem propertyType)
@@ -115,7 +116,7 @@ type UnionConverter<'T>() =
         writer.WriteStringValue(case.Name)
 
         match fieldInfos with
-        | [| fi |] ->
+        | [| fi |] when not (Union.typeIsUnionWithConverterAttribute fi.PropertyType) ->
             match fieldValues.[0] with
             | null when options.IgnoreNullValues -> ()
             | fv ->
diff --git a/tests/FsCodec.NewtonsoftJson.Tests/UnionConverterTests.fs b/tests/FsCodec.NewtonsoftJson.Tests/UnionConverterTests.fs
index 4f22aace..a188a817 100644
--- a/tests/FsCodec.NewtonsoftJson.Tests/UnionConverterTests.fs
+++ b/tests/FsCodec.NewtonsoftJson.Tests/UnionConverterTests.fs
@@ -328,6 +328,7 @@ module ``Unmatched case handling`` =
         fun (e : System.InvalidOperationException) -> <@ -1 <> e.Message.IndexOf "No case defined for 'CaseUnknown', and no catchAllCase nominated" @>
         |> raisesWith <@ act() @>
 
+    [<RequireQualifiedAccess>]
     [<JsonConverter(typeof<UnionConverter>, "case", "Catchall")>]
     type DuWithCatchAll =
         | Known
@@ -338,7 +339,7 @@ module ``Unmatched case handling`` =
         let aJson = """{"case":"CaseUnknown"}"""
         let a = JsonConvert.DeserializeObject<DuWithCatchAll>(aJson, settings)
 
-        test <@ Catchall = a @>
+        test <@ DuWithCatchAll.Catchall = a @>
 
     [<JsonConverter(typeof<UnionConverter>, "case", "CatchAllThatCantBeFound")>]
     type DuWithMissingCatchAll =
diff --git a/tests/FsCodec.SystemTextJson.Tests/SerdesTests.fs b/tests/FsCodec.SystemTextJson.Tests/SerdesTests.fs
index 935c9320..c2f7238d 100644
--- a/tests/FsCodec.SystemTextJson.Tests/SerdesTests.fs
+++ b/tests/FsCodec.SystemTextJson.Tests/SerdesTests.fs
@@ -39,7 +39,7 @@ module StjCharacterization =
 
         let correctSer = """["A,"B"]"""
         raisesWith <@ Serdes.Deserialize<string list>(correctSer, ootbOptions) @>
-            <| fun e -> <@ e.Message.Contains "Deserialization of reference types without a parameterless constructor, a singular parameterized constructor, or a parameterized constructor annotated with 'JsonConstructorAttribute' is not supported. Type 'Microsoft.FSharp.Collections.FSharpList`1[System.String]" @>
+            <| fun e -> <@ e.Message.Contains "s abstract, an interface, or is read only, and could not be instantiated and populated" @>
 
     // System.Text.Json's JsonSerializerOptions by default escapes HTML-sensitive characters when generating JSON strings
     // while this arguably makes sense as a default
diff --git a/tests/FsCodec.SystemTextJson.Tests/UnionConverterTests.fs b/tests/FsCodec.SystemTextJson.Tests/UnionConverterTests.fs
index 5ef52b72..f5109de1 100644
--- a/tests/FsCodec.SystemTextJson.Tests/UnionConverterTests.fs
+++ b/tests/FsCodec.SystemTextJson.Tests/UnionConverterTests.fs
@@ -251,22 +251,17 @@ module ``Unmatched case handling`` =
         |> raisesWith <@ act() @>
 
     [<RequireQualifiedAccess;
-      JsonConverter(typeof<UnionConverter<DuWithCatchAllWithAttributes>>); JsonUnionConverterOptions("case", CatchAllCase = "Catchall")>]
-    type DuWithCatchAllWithAttributes =
+      JsonConverter(typeof<UnionConverter<DuWithCatchAll>>); JsonUnionConverterOptions("case", CatchAllCase = "Catchall")>]
+    type DuWithCatchAll =
         | Known
         | Catchall
 
     [<Fact>]
     let ``UnionConverter supports a nominated catchall via attributes`` () =
         let aJson = """{"case":"CaseUnknown"}"""
-        let a = JsonSerializer.Deserialize<DuWithCatchAllWithAttributes>(aJson)
+        let a = JsonSerializer.Deserialize<DuWithCatchAll>(aJson)
 
-        test <@ DuWithCatchAllWithAttributes.Catchall = a @>
-
-    [<RequireQualifiedAccess>]
-    type DuWithCatchAllWithoutAttributes =
-        | Known
-        | Catchall
+        test <@ DuWithCatchAll.Catchall = a @>
 
     [<RequireQualifiedAccess;
       JsonConverter(typeof<UnionConverter<DuWithMissingCatchAll>>); JsonUnionConverterOptions("case", CatchAllCase = "CatchAllThatCantBeFound")>]
@@ -376,3 +371,122 @@ module ``Struct discriminated unions`` =
 
         let i = CaseIV ( {test = "hi" }, "bye")
         test <@ """{"case":"CaseIV","iv":{"test":"hi"},"ibv":"bye"}""" = serialize i @>
+
+module Nested =
+
+    [<JsonConverter(typeof<UnionConverter<U>>)>]
+    type U =
+        | B of NU
+        | C of UUA
+        | D of UU
+        | E of E
+        | EA of E[]
+        | R of {| a : int; b : NU |}
+        | S
+    and [<JsonConverter(typeof<UnionConverter<NU>>)>]
+        NU =
+        | A of string
+        | B of int
+        | R of {| a : int; b : NU |}
+        | S
+    and [<JsonConverter(typeof<UnionConverter<UU>>)>]
+        UU =
+        | A of string
+        | B of int
+        | E of E
+        | EO of E option
+        | R of {| a: int; b: string |}
+        | S
+    and [<JsonConverter(typeof<UnionConverter<UUA>>); JsonUnionConverterOptions("case2")>]
+        UUA =
+        | A of string
+        | B of int
+        | E of E
+        | EO of E option
+        | R of {| a: int; b: string |}
+        | S
+    and [<JsonConverter(typeof<TypeSafeEnumConverter<E>>)>]
+        E =
+        | V1
+        | V2
+
+    let ro = Options.Create()
+    do ro.IgnoreReadOnlyFields <- true
+    do ro.IgnoreReadOnlyProperties <- true
+
+    let [<FsCheck.Xunit.Property>] ``can nest`` (value : U) =
+        let ser = Serdes.Serialize(value, ro)
+        test <@ value = Serdes.Deserialize(ser, ro) @>
+
+    let [<Fact>] ``nesting Unions represents child as item`` () =
+        let v : U = U.C (UUA.B 42)
+        let ser = Serdes.Serialize(v, ro)
+        """{"case":"C","Item":{"case2":"B","Item":42}}""" =! ser
+        test <@ v = Serdes.Deserialize(ser, ro) @>
+
+    let [<Fact>] ``TypeSafeEnum converts direct`` () =
+        let v : U = U.C (UUA.E E.V1)
+        let ser = Serdes.Serialize(v, ro)
+        """{"case":"C","Item":{"case2":"E","Item":"V1"}}""" =! ser
+        test <@ v = Serdes.Deserialize(ser, ro) @>
+
+        let v : U = U.E E.V2
+        let ser = Serdes.Serialize v
+        """{"case":"E","Item":"V2"}""" =! ser
+        test <@ v = Serdes.Deserialize ser @>
+
+        let v : U = U.EA [|E.V2; E.V2|]
+        let ser = Serdes.Serialize v
+        """{"case":"EA","Item":["V2","V2"]}""" =! ser
+        test <@ v = Serdes.Deserialize ser @>
+
+        let v : U = U.C (UUA.EO (Some E.V1))
+        let ser = Serdes.Serialize v
+        """{"case":"C","Item":{"case2":"EO","Item":"V1"}}""" =! ser
+        test <@ v = Serdes.Deserialize ser @>
+
+        let v : U = U.C (UUA.EO None)
+        let ser = Serdes.Serialize v
+        """{"case":"C","Item":{"case2":"EO","Item":null}}""" =! ser
+        test <@ v = Serdes.Deserialize ser @>
+
+        let v : U = U.C UUA.S
+        let ser = Serdes.Serialize v
+        """{"case":"C","Item":{"case2":"S"}}""" =! ser
+        test <@ v = Serdes.Deserialize ser @>
+
+/// And for everything else, JsonIsomorphism allows plenty ways of customizing the encoding and/or decoding
+module IsomorphismUnionEncoder =
+
+    type [<JsonConverter(typeof<TopConverter>)>]
+        Top =
+        | S
+        | N of Nested
+    and Nested =
+        | A
+        | B of int
+    and TopConverter() =
+        inherit JsonIsomorphism<Top, Flat<int>>()
+        override __.Pickle value =
+            match value with
+            | S -> { disc = TS; v = None }
+            | N A -> { disc = TA; v = None }
+            | N (B v) -> { disc = TB; v = Some v }
+        override __.UnPickle flat =
+            match flat with
+            | { disc = TS } -> S
+            | { disc = TA } -> N A
+            | { disc = TB; v = v} -> N (B (Option.get v))
+    and Flat<'T> = { disc : JiType; v : 'T option }
+    and [<JsonConverter(typeof<TypeSafeEnumConverter<JiType>>)>]
+        JiType = TS | TA | TB
+
+    let [<Fact>] ``Can control the encoding to the nth degree`` () =
+        let v : Top = N (B 42)
+        let ser = Serdes.Serialize v
+        """{"disc":"TB","v":42}""" =! ser
+        test <@ v = Serdes.Deserialize ser @>
+
+    let [<FsCheck.Xunit.Property>] ``can roundtrip`` (value : Top) =
+        let ser = Serdes.Serialize value
+        test <@ value = Serdes.Deserialize ser @>