diff --git a/pkg/compiler/codegen.go b/pkg/compiler/codegen.go
index 41b6aa963..0d4f59a17 100644
--- a/pkg/compiler/codegen.go
+++ b/pkg/compiler/codegen.go
@@ -783,7 +783,7 @@ func (c *codegen) Visit(node ast.Node) ast.Visitor {
 			ln := len(n.Elts)
 			// ByteArrays needs a different approach than normal arrays.
 			if isByteSlice(typ) {
-				c.convertByteArray(n)
+				c.convertByteArray(n.Elts)
 				return nil
 			}
 			for i := ln - 1; i >= 0; i-- {
@@ -1621,20 +1621,46 @@ func (c *codegen) convertBuiltin(expr *ast.CallExpr) {
 	case "append":
 		arg := expr.Args[0]
 		typ := c.typeInfo.Types[arg].Type
-		c.emitReverse(len(expr.Args))
+		ast.Walk(c, arg)
 		emit.Opcodes(c.prog.BinWriter, opcode.DUP, opcode.ISNULL)
-		emit.Instruction(c.prog.BinWriter, opcode.JMPIFNOT, []byte{2 + 3})
 		if isByteSlice(typ) {
-			emit.Opcodes(c.prog.BinWriter, opcode.DROP, opcode.PUSH0, opcode.NEWBUFFER)
-		} else {
-			emit.Opcodes(c.prog.BinWriter, opcode.DROP, opcode.NEWARRAY0, opcode.NOP)
-		}
-		// Jump target.
-		for range expr.Args[1:] {
-			if isByteSlice(typ) {
-				emit.Opcodes(c.prog.BinWriter, opcode.SWAP, opcode.CAT)
+			emit.Instruction(c.prog.BinWriter, opcode.JMPIFNOT, []byte{2 + 3})
+			emit.Opcodes(c.prog.BinWriter, opcode.DROP, opcode.PUSHDATA1, 0)
+			if expr.Ellipsis.IsValid() {
+				ast.Walk(c, expr.Args[1])
 			} else {
-				emit.Opcodes(c.prog.BinWriter, opcode.DUP, opcode.ROT, opcode.APPEND)
+				c.convertByteArray(expr.Args[1:])
+			}
+			emit.Opcodes(c.prog.BinWriter, opcode.CAT)
+		} else {
+			emit.Instruction(c.prog.BinWriter, opcode.JMPIFNOT, []byte{2 + 2})
+			emit.Opcodes(c.prog.BinWriter, opcode.DROP, opcode.NEWARRAY0)
+			if expr.Ellipsis.IsValid() {
+				ast.Walk(c, expr.Args[1])                    // x y
+				emit.Opcodes(c.prog.BinWriter, opcode.PUSH0) // x y cnt=0
+				start := c.newLabel()
+				c.setLabel(start)
+				emit.Opcodes(c.prog.BinWriter, opcode.PUSH2, opcode.PICK) // x y cnt x
+				emit.Opcodes(c.prog.BinWriter, opcode.PUSH2, opcode.PICK) // x y cnt x y
+				emit.Opcodes(c.prog.BinWriter, opcode.DUP, opcode.SIZE)   // x y cnt x y len(y)
+				emit.Opcodes(c.prog.BinWriter, opcode.PUSH3, opcode.PICK) // x y cnt x y len(y) cnt
+				after := c.newLabel()
+				emit.Jmp(c.prog.BinWriter, opcode.JMPEQL, after)          // x y cnt x y
+				emit.Opcodes(c.prog.BinWriter, opcode.PUSH2, opcode.PICK, // x y cnt x y cnt
+					opcode.PICKITEM, // x y cnt x y[cnt]
+					opcode.APPEND,   // x=append(x, y[cnt]) y cnt
+					opcode.INC)      // x y cnt+1
+				emit.Jmp(c.prog.BinWriter, opcode.JMPL, start)
+				c.setLabel(after)
+				for i := 0; i < 4; i++ { // leave x on stack
+					emit.Opcodes(c.prog.BinWriter, opcode.DROP)
+				}
+			} else {
+				for _, e := range expr.Args[1:] {
+					emit.Opcodes(c.prog.BinWriter, opcode.DUP)
+					ast.Walk(c, e)
+					emit.Opcodes(c.prog.BinWriter, opcode.APPEND)
+				}
 			}
 		}
 	case "panic":
@@ -1696,7 +1722,7 @@ func transformArgs(fun ast.Expr, args []ast.Expr) []ast.Expr {
 		}
 	case *ast.Ident:
 		switch f.Name {
-		case "make", "copy":
+		case "make", "copy", "append":
 			return nil
 		}
 	}
@@ -1709,11 +1735,11 @@ func (c *codegen) emitConvert(typ stackitem.Type) {
 	emit.Instruction(c.prog.BinWriter, opcode.CONVERT, []byte{byte(typ)})
 }
 
-func (c *codegen) convertByteArray(lit *ast.CompositeLit) {
-	buf := make([]byte, len(lit.Elts))
+func (c *codegen) convertByteArray(elems []ast.Expr) {
+	buf := make([]byte, len(elems))
 	varIndices := []int{}
-	for i := 0; i < len(lit.Elts); i++ {
-		t := c.typeAndValueOf(lit.Elts[i])
+	for i := 0; i < len(elems); i++ {
+		t := c.typeAndValueOf(elems[i])
 		if t.Value != nil {
 			val, _ := constant.Int64Val(t.Value)
 			buf[i] = byte(val)
@@ -1726,7 +1752,7 @@ func (c *codegen) convertByteArray(lit *ast.CompositeLit) {
 	for _, i := range varIndices {
 		emit.Opcodes(c.prog.BinWriter, opcode.DUP)
 		emit.Int(c.prog.BinWriter, int64(i))
-		ast.Walk(c, lit.Elts[i])
+		ast.Walk(c, elems[i])
 		emit.Opcodes(c.prog.BinWriter, opcode.SETITEM)
 	}
 }
diff --git a/pkg/compiler/slice_test.go b/pkg/compiler/slice_test.go
index 95e667235..37f163b38 100644
--- a/pkg/compiler/slice_test.go
+++ b/pkg/compiler/slice_test.go
@@ -181,6 +181,23 @@ var sliceTestCases = []testCase{
 			stackitem.NewBigInteger(big.NewInt(5)),
 		},
 	},
+	{
+		"int slice, append slice",
+		`package foo
+			func getByte() byte { return 0x80 }
+			func Main() []int {
+				x := []int{1}
+				y := []int{2, 3}
+				x = append(x, y...)
+				x = append(x, y...)
+				return x
+			}`,
+		[]stackitem.Item{
+			stackitem.Make(1),
+			stackitem.Make(2), stackitem.Make(3),
+			stackitem.Make(2), stackitem.Make(3),
+		},
+	},
 	{
 		"declare compound slice",
 		`package foo
@@ -336,6 +353,74 @@ func TestSliceOperations(t *testing.T) {
 	runTestCases(t, sliceTestCases)
 }
 
+func TestByteSlices(t *testing.T) {
+	testCases := []testCase{
+		{
+			"append bytes bigger than 0x79",
+			`package foo
+			func Main() []byte {
+				var z []byte
+				z = append(z, 0x78, 0x79, 0x80, 0x81, 0xFF)
+				return z
+			}`,
+			[]byte{0x78, 0x79, 0x80, 0x81, 0xFF},
+		},
+		{
+			"append bytes bigger than 0x79, not nil",
+			`package foo
+			func Main() []byte {
+				z := []byte{0x78}
+				z = append(z, 0x79, 0x80, 0x81, 0xFF)
+				return z
+			}`,
+			[]byte{0x78, 0x79, 0x80, 0x81, 0xFF},
+		},
+		{
+			"append bytes bigger than 0x79, function return",
+			`package foo
+			func getByte() byte { return 0x80 }
+			func Main() []byte {
+				var z []byte
+				z = append(z, 0x78, 0x79, getByte(), 0x81, 0xFF)
+				return z
+			}`,
+			[]byte{0x78, 0x79, 0x80, 0x81, 0xFF},
+		},
+		{
+			"append ints bigger than 0x79, function return byte",
+			`package foo
+			func getByte() byte { return 0x80 }
+			func Main() []int {
+				var z []int
+				z = append(z, 0x78, int(getByte()))
+				return z
+			}`,
+			[]stackitem.Item{stackitem.Make(0x78), stackitem.Make(0x80)},
+		},
+		{
+			"slice literal, bytes bigger than 0x79, function return",
+			`package foo
+			func getByte() byte { return 0x80 }
+			func Main() []byte {
+				z := []byte{0x79, getByte(), 0x81}
+				return z
+			}`,
+			[]byte{0x79, 0x80, 0x81},
+		},
+		{
+			"compare bytes as integers",
+			`package foo
+				func getByte1() byte { return 0x79 }
+				func getByte2() byte { return 0x80 }
+				func Main() bool {
+					return getByte1() < getByte2()
+				}`,
+			true,
+		},
+	}
+	runTestCases(t, testCases)
+}
+
 func TestSubsliceCompound(t *testing.T) {
 	src := `package foo
 	func Main() []int {