diff --git a/src/stdlib/Conversions.scillib b/src/stdlib/Conversions.scillib
index 06b358652..26bfc96d6 100644
--- a/src/stdlib/Conversions.scillib
+++ b/src/stdlib/Conversions.scillib
@@ -206,7 +206,7 @@ let append_uint128 : IntegerEncoding -> ByStr -> Uint128 -> ByStr =
     let uib_endian =
       match endian with | LittleEndian => builtin strrev uib | BigEndian => uib end
     in
-    builtin concat bs uib
+    builtin concat bs uib_endian
 
 (* Append serialized Uint256 value to given byte string *)
 let append_uint256 : IntegerEncoding -> ByStr -> Uint256 -> ByStr =
@@ -218,4 +218,4 @@ let append_uint256 : IntegerEncoding -> ByStr -> Uint256 -> ByStr =
     let uib_endian =
       match endian with | LittleEndian => builtin strrev uib | BigEndian => uib end
     in
-    builtin concat bs uib
+    builtin concat bs uib_endian
diff --git a/tests/eval/good/Good.ml b/tests/eval/good/Good.ml
index 6499cbba4..51e91aaea 100644
--- a/tests/eval/good/Good.ml
+++ b/tests/eval/good/Good.ml
@@ -150,6 +150,10 @@ let explist =
     "polynetwork_getBookKeeper.scilexp";
     "polynetwork_txparam.scilexp";
     "builtin_type_args.scilexp";
+    "endian_test32.scilexp";
+    "endian_test64.scilexp";
+    "endian_test128.scilexp";
+    "endian_test256.scilexp";
   ]
 
 module Tests = Scilla_test.Util.DiffBasedTests (struct
diff --git a/tests/eval/good/endian_test128.scilexp b/tests/eval/good/endian_test128.scilexp
new file mode 100644
index 000000000..515bdf9d1
--- /dev/null
+++ b/tests/eval/good/endian_test128.scilexp
@@ -0,0 +1,15 @@
+let i = Uint128 12345 in
+let empty_x = 0x in
+
+let empty = builtin to_bystr empty_x in
+
+let ile = append_uint128_le empty i in
+
+let big_endian = BigEndian in
+let extract_uint128_be = extract_uint128 big_endian in
+let append_uint128_be = append_uint128 big_endian in
+
+let ibe = append_uint128_be empty i in
+
+Pair {ByStr ByStr} ile ibe
+
diff --git a/tests/eval/good/endian_test256.scilexp b/tests/eval/good/endian_test256.scilexp
new file mode 100644
index 000000000..e2181f202
--- /dev/null
+++ b/tests/eval/good/endian_test256.scilexp
@@ -0,0 +1,15 @@
+let i = Uint256 12345 in
+let empty_x = 0x in
+
+let empty = builtin to_bystr empty_x in
+
+let ile = append_uint256_le empty i in
+
+let big_endian = BigEndian in
+let extract_uint256_be = extract_uint256 big_endian in
+let append_uint256_be = append_uint256 big_endian in
+
+let ibe = append_uint256_be empty i in
+
+Pair {ByStr ByStr} ile ibe
+
diff --git a/tests/eval/good/endian_test32.scilexp b/tests/eval/good/endian_test32.scilexp
new file mode 100644
index 000000000..c3eda25b3
--- /dev/null
+++ b/tests/eval/good/endian_test32.scilexp
@@ -0,0 +1,15 @@
+let i = Uint32 12345 in
+let empty_x = 0x in
+
+let empty = builtin to_bystr empty_x in
+
+let ile = append_uint32_le empty i in
+
+let big_endian = BigEndian in
+let extract_uint32_be = extract_uint32 big_endian in
+let append_uint32_be = append_uint32 big_endian in
+
+let ibe = append_uint32_be empty i in
+
+Pair {ByStr ByStr} ile ibe
+
diff --git a/tests/eval/good/endian_test64.scilexp b/tests/eval/good/endian_test64.scilexp
new file mode 100644
index 000000000..56757d446
--- /dev/null
+++ b/tests/eval/good/endian_test64.scilexp
@@ -0,0 +1,15 @@
+let i = Uint64 12345 in
+let empty_x = 0x in
+
+let empty = builtin to_bystr empty_x in
+
+let ile = append_uint64_le empty i in
+
+let big_endian = BigEndian in
+let extract_uint64_be = extract_uint64 big_endian in
+let append_uint64_be = append_uint64 big_endian in
+
+let ibe = append_uint64_be empty i in
+
+Pair {ByStr ByStr} ile ibe
+
diff --git a/tests/eval/good/gold/endian_test128.scilexp.gold b/tests/eval/good/gold/endian_test128.scilexp.gold
new file mode 100644
index 000000000..88fb2cc47
--- /dev/null
+++ b/tests/eval/good/gold/endian_test128.scilexp.gold
@@ -0,0 +1,10 @@
+(Pair (ByStr 0x39300000000000000000000000000000) (ByStr 0x00000000000000000000000000003039)),
+{ [ibe -> (ByStr 0x00000000000000000000000000003039)],
+  [append_uint128_be -> <closure>],
+  [extract_uint128_be -> <closure>],
+  [big_endian -> (Conversions.BigEndian)],
+  [ile -> (ByStr 0x39300000000000000000000000000000)],
+  [empty -> (ByStr 0x)],
+  [empty_x -> (ByStr0 0x)],
+  [i -> (Uint128 12345)] }
+Gas remaining: 4001096
diff --git a/tests/eval/good/gold/endian_test256.scilexp.gold b/tests/eval/good/gold/endian_test256.scilexp.gold
new file mode 100644
index 000000000..87d14b4e4
--- /dev/null
+++ b/tests/eval/good/gold/endian_test256.scilexp.gold
@@ -0,0 +1,10 @@
+(Pair (ByStr 0x3930000000000000000000000000000000000000000000000000000000000000) (ByStr 0x0000000000000000000000000000000000000000000000000000000000003039)),
+{ [ibe -> (ByStr 0x0000000000000000000000000000000000000000000000000000000000003039)],
+  [append_uint256_be -> <closure>],
+  [extract_uint256_be -> <closure>],
+  [big_endian -> (Conversions.BigEndian)],
+  [ile -> (ByStr 0x3930000000000000000000000000000000000000000000000000000000000000)],
+  [empty -> (ByStr 0x)],
+  [empty_x -> (ByStr0 0x)],
+  [i -> (Uint256 12345)] }
+Gas remaining: 4000984
diff --git a/tests/eval/good/gold/endian_test32.scilexp.gold b/tests/eval/good/gold/endian_test32.scilexp.gold
new file mode 100644
index 000000000..b60eb9084
--- /dev/null
+++ b/tests/eval/good/gold/endian_test32.scilexp.gold
@@ -0,0 +1,10 @@
+(Pair (ByStr 0x39300000) (ByStr 0x00003039)),
+{ [ibe -> (ByStr 0x00003039)],
+  [append_uint32_be -> <closure>],
+  [extract_uint32_be -> <closure>],
+  [big_endian -> (Conversions.BigEndian)],
+  [ile -> (ByStr 0x39300000)],
+  [empty -> (ByStr 0x)],
+  [empty_x -> (ByStr0 0x)],
+  [i -> (Uint32 12345)] }
+Gas remaining: 4001180
diff --git a/tests/eval/good/gold/endian_test64.scilexp.gold b/tests/eval/good/gold/endian_test64.scilexp.gold
new file mode 100644
index 000000000..be1063157
--- /dev/null
+++ b/tests/eval/good/gold/endian_test64.scilexp.gold
@@ -0,0 +1,10 @@
+(Pair (ByStr 0x3930000000000000) (ByStr 0x0000000000003039)),
+{ [ibe -> (ByStr 0x0000000000003039)],
+  [append_uint64_be -> <closure>],
+  [extract_uint64_be -> <closure>],
+  [big_endian -> (Conversions.BigEndian)],
+  [ile -> (ByStr 0x3930000000000000)],
+  [empty -> (ByStr 0x)],
+  [empty_x -> (ByStr0 0x)],
+  [i -> (Uint64 12345)] }
+Gas remaining: 4001152