diff --git a/src/io/buffer.rs b/src/io/buffer.rs
index 01b1f91f..ace99dd4 100644
--- a/src/io/buffer.rs
+++ b/src/io/buffer.rs
@@ -8,8 +8,8 @@ use std::iter;
 #[derive(Debug, Default)]
 pub struct BitBuffer {
     buffer: Vec<u8>,
-    write_position: usize,
-    read_position: usize,
+    pub(crate) write_position: usize,
+    pub(crate) read_position: usize,
 }
 
 impl BitBuffer {
@@ -62,6 +62,20 @@ impl BitBuffer {
     pub fn byte_len(&self) -> usize {
         self.buffer.len()
     }
+
+    /// Changes the write-position to the given position for the closure call.
+    /// Restores the original write position after the call.
+    ///
+    /// # Panics
+    /// Positions beyond the current buffer length will result in panics.
+    pub fn with_write_position_at<T, F: Fn(&mut Self) -> T>(&mut self, position: usize, f: F) -> T {
+        assert!(position <= self.buffer.len());
+        let before = self.write_position;
+        self.write_position = position;
+        let result = f(self);
+        self.write_position = before;
+        result
+    }
 }
 
 fn bit_string_copy(
diff --git a/src/syn/io/mod.rs b/src/syn/io/mod.rs
index d49701e5..b2271a64 100644
--- a/src/syn/io/mod.rs
+++ b/src/syn/io/mod.rs
@@ -1,3 +1,5 @@
 mod println;
+mod uper;
 
 pub use println::*;
+pub use uper::*;
diff --git a/src/syn/io/uper.rs b/src/syn/io/uper.rs
new file mode 100644
index 00000000..5f4e1d3c
--- /dev/null
+++ b/src/syn/io/uper.rs
@@ -0,0 +1,75 @@
+use crate::io::buffer::BitBuffer;
+use crate::io::uper::Error as UperError;
+use crate::io::uper::Writer as _;
+use crate::prelude::*;
+use std::fmt::{Display, Formatter};
+
+#[derive(Default)]
+pub struct UperWriter {
+    buffer: BitBuffer,
+    optional_positions: Vec<usize>,
+}
+
+impl UperWriter {
+    pub fn byte_content(&self) -> &[u8] {
+        self.buffer.content()
+    }
+
+    pub const fn bit_len(&self) -> usize {
+        self.buffer.bit_len()
+    }
+}
+
+impl Writer for UperWriter {
+    type Error = UperError;
+
+    fn write_sequence<C: sequence::Constraint, F: Fn(&mut Self) -> Result<(), Self::Error>>(
+        &mut self,
+        f: F,
+    ) -> Result<(), Self::Error> {
+        // in UPER the optional flag for all OPTIONAL values is written before any field
+        // value is written. This reserves the bits, so that on a later call of `write_opt`
+        // the value can be set to the actual state.
+        let before = self.optional_positions.len();
+        let write_pos = self.buffer.write_position;
+        for i in (0..C::OPTIONAL_FIELDS).rev() {
+            // insert in reverse order so that a simple pop() in `write_opt` retrieves
+            // the relevant position
+            self.optional_positions.push(write_pos + i);
+            self.buffer.write_bit(false);
+        }
+        f(self)?;
+        assert_eq!(before, self.optional_positions.len());
+        Ok(())
+    }
+
+    fn write_opt<T: WritableType>(
+        &mut self,
+        value: Option<&<T as WritableType>::Type>,
+    ) -> Result<(), Self::Error> {
+        self.buffer
+            .with_write_position_at(self.optional_positions.pop().unwrap(), |buffer| {
+                buffer.write_bit(value.is_some())
+            })?;
+        if let Some(value) = value {
+            T::write_value(self, value)
+        } else {
+            Ok(())
+        }
+    }
+
+    fn write_int(&mut self, value: i64, range: (i64, i64)) -> Result<(), Self::Error> {
+        self.buffer.write_int(value, range)
+    }
+
+    fn write_int_max(&mut self, value: u64) -> Result<(), Self::Error> {
+        self.buffer.write_int_max(value)
+    }
+
+    fn write_utf8string<C: utf8string::Constraint>(
+        &mut self,
+        value: &str,
+    ) -> Result<(), Self::Error> {
+        self.buffer.write_utf8_string(value)
+    }
+}
diff --git a/tests/basic_proc_macro_attribute.rs b/tests/basic_proc_macro_attribute.rs
index 75eb950c..c7d58469 100644
--- a/tests/basic_proc_macro_attribute.rs
+++ b/tests/basic_proc_macro_attribute.rs
@@ -1,5 +1,6 @@
 #![allow(unused)]
 
+use asn1rs::syn::io::UperWriter;
 use asn1rs_macros::asn;
 
 #[asn(sequence)]
@@ -24,3 +25,24 @@ fn test_compiles() {
         string: String::from("where is the content"),
     };
 }
+
+#[test]
+fn test_serialize_with_uper() {
+    let p = Potato {
+        size: 123,
+        size2: 1234,
+        size3: 128,
+        string: String::from("where is the content"),
+    };
+    let mut uper = UperWriter::default();
+    uper.write(&p).unwrap();
+    assert_eq!(
+        &[
+            // https://asn1.io/asn1playground/
+            0x01, 0x7B, 0x02, 0x04, 0xD2, 0xE8, 0x28, 0xEE, 0xD0, 0xCA, 0xE4, 0xCA, 0x40, 0xD2,
+            0xE6, 0x40, 0xE8, 0xD0, 0xCA, 0x40, 0xC6, 0xDE, 0xDC, 0xE8, 0xCA, 0xDC, 0xE8
+        ],
+        uper.byte_content()
+    );
+    assert_eq!(26 * 8 + 7, uper.bit_len());
+}