diff --git a/conversion.go b/conversion.go
index c7da0c73..ad25895f 100644
--- a/conversion.go
+++ b/conversion.go
@@ -77,7 +77,7 @@ func FromBig(b *big.Int) (*Int, bool) {
 
 // MustFromBig is a convenience-constructor from big.Int.
 // Returns a new Int and panics if overflow occurred.
-// OBS: If b is `nil`, this method does _not_ panic, but 
+// OBS: If b is `nil`, this method does _not_ panic, but
 // instead returns `nil`
 func MustFromBig(b *big.Int) *Int {
 	if b == nil {
@@ -135,7 +135,6 @@ func (z *Int) Float64() float64 {
 // - This method does not accept negative zero as valid, e.g "-0x0",
 //   - (this method does not accept any negative input as valid)
 func (z *Int) SetFromHex(hex string) error {
-	z.Clear()
 	return z.fromHex(hex)
 }
 
@@ -147,6 +146,7 @@ func (z *Int) fromHex(hex string) error {
 	if len(hex) > 66 {
 		return ErrBig256Range
 	}
+	z.Clear()
 	end := len(hex)
 	for i := 0; i < 4; i++ {
 		start := end - 16
@@ -188,10 +188,14 @@ func MustFromHex(hex string) *Int {
 	return &z
 }
 
-// UnmarshalText implements encoding.TextUnmarshaler
+// UnmarshalText implements encoding.TextUnmarshaler. This method
+// can unmarshal either hexadecimal or decimal.
+// - For hexadecimal, the input _must_ be prefixed with 0x or 0X
 func (z *Int) UnmarshalText(input []byte) error {
-	z.Clear()
-	return z.fromHex(string(input))
+	if len(input) >= 2 && input[0] == '0' && (input[1] == 'x' || input[1] == 'X') {
+		return z.fromHex(string(input))
+	}
+	return z.fromDecimal(string(input))
 }
 
 // SetFromBig converts a big.Int to Int and sets the value to z.
@@ -613,26 +617,36 @@ func (z *Int) EncodeRLP(w io.Writer) error {
 }
 
 // MarshalText implements encoding.TextMarshaler
+// MarshalText marshals using the decimal representation (compatible with big.Int)
 func (z *Int) MarshalText() ([]byte, error) {
-	return []byte(z.Hex()), nil
+	return []byte(z.Dec()), nil
 }
 
 // MarshalJSON implements json.Marshaler.
+// MarshalJSON marshals using the 'decimal string' representation. This is _not_ compatible
+// with big.Int: big.Int marshals into JSON 'native' numeric format.
+//
+// The JSON  native format is, on some platforms, (e.g. javascript), limited to 53-bit large
+// integer space. Thus, U256 uses string-format, which is not compatible with
+// big.int (big.Int refuses to unmarshal a string representation).
 func (z *Int) MarshalJSON() ([]byte, error) {
-	return []byte(`"` + z.Hex() + `"`), nil
+	return []byte(`"` + z.Dec() + `"`), nil
 }
 
-// UnmarshalJSON implements json.Unmarshaler.
+// UnmarshalJSON implements json.Unmarshaler. UnmarshalJSON accepts either
+// - Quoted string: either hexadecimal OR decimal
+// - Not quoted string: only decimal
 func (z *Int) UnmarshalJSON(input []byte) error {
 	if len(input) < 2 || input[0] != '"' || input[len(input)-1] != '"' {
-		return ErrNonString
+		// if not quoted, it must be decimal
+		return z.fromDecimal(string(input))
 	}
 	return z.UnmarshalText(input[1 : len(input)-1])
 }
 
-// String returns the hex encoding of b.
+// String returns the decimal encoding of b.
 func (z *Int) String() string {
-	return z.Hex()
+	return z.Dec()
 }
 
 const (
@@ -738,7 +752,6 @@ var (
 	ErrEmptyNumber      = errors.New("hex string \"0x\"")
 	ErrLeadingZero      = errors.New("hex number with leading zero digits")
 	ErrBig256Range      = errors.New("hex number > 256 bits")
-	ErrNonString        = errors.New("non-string")
 	ErrBadBufferLength  = errors.New("bad ssz buffer length")
 	ErrBadEncodedLength = errors.New("bad ssz encoded length")
 )
diff --git a/conversion_test.go b/conversion_test.go
index 05451add..0f8864e2 100644
--- a/conversion_test.go
+++ b/conversion_test.go
@@ -1196,10 +1196,12 @@ func TestDecode(t *testing.T) {
 		Foo *Int
 	}
 	var jsonDecoded jsonStruct
-	if err := json.Unmarshal([]byte(`{"Foo":0x1}`), &jsonDecoded); err == nil {
-		t.Fatal("Expected error")
+	// This test was previously an "expected error", The U256 behaviour has now
+	// changed, to be compatible with big.Int
+	if err := json.Unmarshal([]byte(`{"Foo":1}`), &jsonDecoded); err != nil {
+		t.Fatalf("Expected no error, have %v", err)
 	}
-	if err := json.Unmarshal([]byte(`{"Foo":1}`), &jsonDecoded); err == nil {
+	if err := json.Unmarshal([]byte(`{"Foo":0x1}`), &jsonDecoded); err == nil {
 		t.Fatal("Expected error")
 	}
 	if err := json.Unmarshal([]byte(`{"Foo":""}`), &jsonDecoded); err == nil {
@@ -1216,38 +1218,61 @@ func TestEnDecode(t *testing.T) {
 	type jsonStruct struct {
 		Foo *Int
 	}
+	type jsonBigStruct struct {
+		Foo *big.Int
+	}
 	var testSample = func(i int, bigSample big.Int, intSample Int) {
 		// Encoding
 		wantHex := fmt.Sprintf("0x%s", bigSample.Text(16))
 		wantDec := bigSample.Text(10)
 
-		if got := intSample.Hex(); wantHex != got {
-			t.Fatalf("test %d #1, got %v, exp %v", i, got, wantHex)
+		if have, want := intSample.Hex(), fmt.Sprintf("0x%s", bigSample.Text(16)); have != want {
+			t.Fatalf("test %d #1, have %v, want %v", i, have, want)
 		}
-		if got := intSample.String(); wantHex != got {
-			t.Fatalf("test %d #2, got %v, exp %v", i, got, wantHex)
+		if have, want := intSample.String(), bigSample.String(); have != want {
+			t.Fatalf("test %d String(), have %v, want %v", i, have, want)
 		}
-		if got, _ := intSample.MarshalText(); wantHex != string(got) {
-			t.Fatalf("test %d #3, got %v, exp %v", i, got, wantHex)
+		{
+			have, _ := intSample.MarshalText()
+			want, _ := bigSample.MarshalText()
+			if !bytes.Equal(have, want) {
+				t.Fatalf("test %d MarshalText, have %q, want %q", i, have, want)
+			}
 		}
-		if got, _ := intSample.Value(); wantDec != got.(string) {
-			t.Fatalf("test %d #4, got %v, exp %v", i, got, wantHex)
+		{
+			have, _ := intSample.MarshalJSON()
+			want := []byte(fmt.Sprintf(`"%s"`, bigSample.Text(10)))
+			if !bytes.Equal(have, want) {
+				t.Fatalf("test %d MarshalJSON, have %q, want %q", i, have, want)
+			}
+		}
+		if have, _ := intSample.Value(); wantDec != have.(string) {
+			t.Fatalf("test %d #4, got %v, exp %v", i, have, wantHex)
 		}
-		if got := intSample.Dec(); wantDec != got {
-			t.Fatalf("test %d #5, got %v, exp %v", i, got, wantHex)
+		if have, want := intSample.Dec(), wantDec; have != want {
+			t.Fatalf("test %d Dec(), have %v, want %v", i, have, want)
 		}
 		{ // Json
 			jsonEncoded, err := json.Marshal(&jsonStruct{&intSample})
 			if err != nil {
-				t.Fatalf("test %d #6, err: %v", i, err)
+				t.Fatalf("test %d: json encoding err: %v", i, err)
 			}
+			jsonEncodedBig, _ := json.Marshal(&jsonBigStruct{&bigSample})
 			var jsonDecoded jsonStruct
 			err = json.Unmarshal(jsonEncoded, &jsonDecoded)
 			if err != nil {
-				t.Fatalf("test %d #7, err: %v", i, err)
+				t.Fatalf("test %d error unmarshaling: %v", i, err)
+			}
+			if jsonDecoded.Foo.Cmp(&intSample) != 0 {
+				t.Fatalf("test %d #8, have %v, want %v", i, jsonDecoded.Foo, intSample)
+			}
+			// See if we can also unmarshal from big.Int's non-string format
+			err = json.Unmarshal(jsonEncodedBig, &jsonDecoded)
+			if err != nil {
+				t.Fatalf("test %d unmarshalling from big.Int err: %v", i, err)
 			}
 			if jsonDecoded.Foo.Cmp(&intSample) != 0 {
-				t.Fatalf("test %d #8, got %v, exp %v", i, jsonDecoded.Foo, intSample)
+				t.Fatalf("test %d have %v, want %v", i, jsonDecoded.Foo, intSample)
 			}
 		}
 		// Decoding