diff --git a/CHANGELOG.md b/CHANGELOG.md
index 8760b4cc..eb6b9e91 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,13 +1,14 @@
 # Changelog
 
-## v1.0.25-alpha - 
+## v1.0.25-alpha - UNRELEASED
 
 ### Added
 
-- **aiken-lang**: Data now has a generic argument that can be used to specify the blueprint type. @KtorZ 
+- **aiken-lang**: Data now has a generic argument that can be used to specify the blueprint type. @KtorZ
 
 ### Fixed
 
+- **uplc**: `serialise_data` builtin wrongly encoding some larger ints as tagged CBOR bigints, instead of plain integers over 9 bytes. @KtorZ
 
 ### Changed
 
diff --git a/crates/uplc/src/ast.rs b/crates/uplc/src/ast.rs
index 1fd3736a..e4e502cd 100644
--- a/crates/uplc/src/ast.rs
+++ b/crates/uplc/src/ast.rs
@@ -281,9 +281,9 @@ impl Data {
         hex::encode(bytes)
     }
     pub fn integer(i: BigInt) -> PlutusData {
-        match i.to_i64() {
-            Some(i) => PlutusData::BigInt(alonzo::BigInt::Int(i.into())),
-            None => {
+        match i.to_i128().map(|n| n.try_into()) {
+            Some(Ok(i)) => PlutusData::BigInt(alonzo::BigInt::Int(i)),
+            _ => {
                 let (sign, bytes) = i.to_bytes_be();
                 match sign {
                     num_bigint::Sign::Minus => {
diff --git a/crates/uplc/src/machine/value.rs b/crates/uplc/src/machine/value.rs
index 37cac1fc..4b8a3228 100644
--- a/crates/uplc/src/machine/value.rs
+++ b/crates/uplc/src/machine/value.rs
@@ -410,10 +410,14 @@ pub fn from_pallas_bigint(n: &babbage::BigInt) -> BigInt {
 }
 
 pub fn to_pallas_bigint(n: &BigInt) -> babbage::BigInt {
-    if let Some(i) = n.to_i64() {
-        let pallas_int: pallas::codec::utils::Int = i.into();
-        babbage::BigInt::Int(pallas_int)
-    } else if n.is_positive() {
+    if let Some(i) = n.to_i128() {
+        if let Ok(i) = i.try_into() {
+            let pallas_int: pallas::codec::utils::Int = i;
+            return babbage::BigInt::Int(pallas_int);
+        }
+    }
+
+    if n.is_positive() {
         let (_, bytes) = n.to_bytes_be();
         babbage::BigInt::BigUInt(bytes.into())
     } else {
diff --git a/examples/acceptance_tests/094/aiken.lock b/examples/acceptance_tests/094/aiken.lock
new file mode 100644
index 00000000..6e350cda
--- /dev/null
+++ b/examples/acceptance_tests/094/aiken.lock
@@ -0,0 +1,7 @@
+# This file was generated by Aiken
+# You typically do not need to edit this file
+
+requirements = []
+packages = []
+
+[etags]
diff --git a/examples/acceptance_tests/094/aiken.toml b/examples/acceptance_tests/094/aiken.toml
new file mode 100644
index 00000000..e933c285
--- /dev/null
+++ b/examples/acceptance_tests/094/aiken.toml
@@ -0,0 +1,3 @@
+name = "aiken-lang/acceptance_test_094"
+version = "0.0.0"
+description = ""
diff --git a/examples/acceptance_tests/094/lib/foo.ak b/examples/acceptance_tests/094/lib/foo.ak
new file mode 100644
index 00000000..54b57b97
--- /dev/null
+++ b/examples/acceptance_tests/094/lib/foo.ak
@@ -0,0 +1,9 @@
+use aiken/builtin
+
+test u32_boundary_down() {
+  builtin.serialise_data(0xdeadbeefdeadbeef) == #"1bdeadbeefdeadbeef"
+}
+
+test u32_boundary_up() {
+  builtin.serialise_data(-0xdeadbeefdeadbeef) == #"3bdeadbeefdeadbeee"
+}