diff --git a/tests/src/Codec/GuidStringCodecTest.php b/tests/src/Codec/GuidStringCodecTest.php
index 69df13e..28132d8 100644
--- a/tests/src/Codec/GuidStringCodecTest.php
+++ b/tests/src/Codec/GuidStringCodecTest.php
@@ -2,6 +2,7 @@
 
 namespace Ramsey\Uuid\Test\Codec;
 
+use Ramsey\Uuid\Codec\GuidStringCodec;
 use Ramsey\Uuid\Test\TestCase;
 
 /**
@@ -12,38 +13,99 @@ use Ramsey\Uuid\Test\TestCase;
 class GuidStringCodecTest extends TestCase
 {
 
+    /** @var UuidBuilderInterface */
+    private $builder;
+    /** @var UuidInterface */
+    private $uuid;
+    /** @var array */
+    private $fields;
+
+    public function setUp()
+    {
+        parent::setUp();
+        $this->builder = $this->getMock('Ramsey\Uuid\Builder\UuidBuilderInterface');
+        $this->uuid = $this->getMock('Ramsey\Uuid\UuidInterface');
+        $this->fields = ['time_low' => '12345678',
+            'time_mid' => '1234',
+            'time_hi_and_version' => 'abcd',
+            'clock_seq_hi_and_reserved' => 'ab',
+            'clock_seq_low' => 'ef',
+            'node' => '1234abcd4321'];
+    }
+
+    public function tearDown()
+    {
+        parent::tearDown();
+        $this->builder = null;
+        $this->fields = null;
+    }
+
     public function testEncodeUsesFieldsArray()
     {
-        
+        $this->uuid->expects($this->once())
+            ->method('getFieldsHex')
+            ->willReturn($this->fields);
+        $codec = new GuidStringCodec($this->builder);
+        $codec->encode($this->uuid);
     }
 
     public function testEncodeReturnsFormattedString()
     {
-
+        $this->uuid->method('getFieldsHex')
+            ->willReturn($this->fields);
+        $codec = new GuidStringCodec($this->builder);
+        $result = $codec->encode($this->uuid);
+        $this->assertEquals('78563412-3412-cdab-abef-1234abcd4321', $result);
     }
 
     public function testEncodeBinaryUsesFieldsArray()
     {
-
+        $this->uuid->expects($this->once())
+            ->method('getFieldsHex')
+            ->willReturn($this->fields);
+        $codec = new GuidStringCodec($this->builder);
+        $codec->encodeBinary($this->uuid);
     }
 
     public function testEncodeBinaryReturnsBinaryString()
     {
-
+        $expected = hex2bin('123456781234abcdabef1234abcd4321');
+        $this->uuid->method('getFieldsHex')
+            ->willReturn($this->fields);
+        $codec = new GuidStringCodec($this->builder);
+        $result = $codec->encodeBinary($this->uuid);
+        $this->assertEquals($expected, $result);
     }
 
     public function testDecodeUsesBuilderOnFields()
     {
-
+        $string = 'uuid:78563412-3412-cdab-abef-1234abcd4321';
+        $this->builder->expects($this->once())
+            ->method('build')
+            ->with($this->isInstanceOf('Ramsey\Uuid\Codec\GuidStringCodec'), $this->fields);
+        $codec = new GuidStringCodec($this->builder);
+        $codec->decode($string);
     }
 
     public function testDecodeReturnsUuidFromBuilder()
     {
+        $string = 'uuid:78563412-3412-cdab-abef-1234abcd4321';
+        $this->builder->method('build')
+            ->willReturn($this->uuid);
 
+        $codec = new GuidStringCodec($this->builder);
+        $result = $codec->decode($string);
+        $this->assertEquals($this->uuid, $result);
     }
 
     public function testDecodeBytesReturnsUuid()
     {
-
+        $string = '123456781234abcdabef1234abcd4321';
+        $bytes = pack('H*', $string);
+        $codec = new GuidStringCodec($this->builder);
+        $this->builder->method('build')
+            ->willReturn($this->uuid);
+        $result = $codec->decodeBytes($bytes);
+        $this->assertEquals($this->uuid, $result);
     }
 }
diff --git a/tests/src/Codec/StringCodecTest.php b/tests/src/Codec/StringCodecTest.php
index 5812fee..fff341f 100644
--- a/tests/src/Codec/StringCodecTest.php
+++ b/tests/src/Codec/StringCodecTest.php
@@ -21,6 +21,8 @@ class StringCodecTest extends TestCase
     private $uuid;
     /** @var array */
     private $fields;
+    /** @var string */
+    private $uuidString = '12345678-1234-abcd-abef-1234abcd4321';
 
     public function setUp()
     {
@@ -55,11 +57,9 @@ class StringCodecTest extends TestCase
     {
         $this->uuid->method('getFieldsHex')
             ->willReturn($this->fields);
-        $expected = '12345678-1234-abcd-abef-1234abcd4321';
-
         $codec = new StringCodec($this->builder);
         $result = $codec->encode($this->uuid);
-        $this->assertEquals($expected, $result);
+        $this->assertEquals($this->uuidString, $result);
     }
 
     public function testEncodeBinaryUsesHexadecimalValue()