diff --git a/exiftool_test.go b/exiftool_test.go
index 27f3528..5dc44dd 100644
--- a/exiftool_test.go
+++ b/exiftool_test.go
@@ -765,3 +765,27 @@ func TestBufferTooSmallError(t *testing.T) {
 	assert.Len(t, fms, 1)
 	assert.Equal(t, ErrBufferTooSmall, fms[0].Err)
 }
+
+func TestWriteMetadataAddString(t *testing.T) {
+	t.Parallel()
+
+	e, err := NewExiftool()
+	assert.Nil(t, err)
+	defer e.Close()
+
+	testFile := filepath.Join(t.TempDir(), "test.jpg")
+	require.Nil(t, copyFile("testdata/20190404_131804.jpg", testFile))
+
+	mds := []FileMetadata{EmptyFileMetadata()}
+	mds[0].File = testFile
+	mds[0].AddString("ImageUniqueID", "newValue")
+	e.WriteMetadata(mds)
+	require.Nil(t, mds[0].Err)
+
+	mds2 := e.ExtractMetadata(testFile)
+	require.Len(t, mds2, 1)
+	require.Nil(t, mds2[0].Err)
+	value, err := mds2[0].GetString("ImageUniqueID")
+	require.Nil(t, err)
+	require.Equal(t, "newValue", value)
+}
diff --git a/filemetadata.go b/filemetadata.go
index 853d06c..91b6054 100644
--- a/filemetadata.go
+++ b/filemetadata.go
@@ -178,3 +178,8 @@ func EmptyFileMetadata() FileMetadata {
 		Fields: make(map[string]interface{}),
 	}
 }
+
+// AddString adds a new key-value pair to the Fields map with a string value
+func (fm *FileMetadata) AddString(key string, value string) {
+	fm.Fields[key] = value
+}
diff --git a/filemetadata_test.go b/filemetadata_test.go
index 0303d28..4375d19 100644
--- a/filemetadata_test.go
+++ b/filemetadata_test.go
@@ -221,3 +221,17 @@ func TestClearAll(t *testing.T) {
 	assert.Equal(t, ErrKeyNotFound, err)
 
 }
+
+func TestAddString(t *testing.T) {
+	fm := EmptyFileMetadata()
+
+	fm.AddString("key", "value")
+	got, err := fm.GetString("key")
+	assert.Nil(t, err)
+	assert.Equal(t, "value", got)
+
+	fm.AddString("key", "new value")
+	got, err = fm.GetString("key")
+	assert.Nil(t, err)
+	assert.Equal(t, "new value", got)
+}