diff --git a/src/main/asciidoc/reference/elasticsearch-migration-guide-5.0-5.1.adoc b/src/main/asciidoc/reference/elasticsearch-migration-guide-5.0-5.1.adoc
index 113b2643a..58482570e 100644
--- a/src/main/asciidoc/reference/elasticsearch-migration-guide-5.0-5.1.adoc
+++ b/src/main/asciidoc/reference/elasticsearch-migration-guide-5.0-5.1.adoc
@@ -6,6 +6,11 @@ This section describes breaking changes from version 5.0.x to 5.1.x and how remo
 [[elasticsearch-migration-guide-5.0-5.1.breaking-changes]]
 == Breaking Changes
 
-In the `org.springframework.data.elasticsearch.core.index.AliasData` class, which is used for alias information 
-returned from Elasticsearch, the property `filter` (of type `Document`) is replaced by `filterQuery` which is of type
+In the `org.springframework.data.elasticsearch.core.index.AliasData` class, which is used for alias information returned from Elasticsearch, the property `filter` (of type `Document`) is replaced by `filterQuery` which is of type
 `org.springframework.data.elasticsearch.core.query.Query`.
+
+`org.springframework.data.elasticsearch.annotations.Similarity` was an enum class until 5.1. This enum was used in the `@Field` annotation to specify a similarity value.
+But besides the values defined by the enum, it is possible to have similarities with custom names in Elasticsearch.
+Therefore, the annotation property was changed from the type of the enum to a simple `String`.
+The previous enum values like `Similarity.Default` do still exist as String constants, so existing code will compile unmodified.
+Adaptions are necessary when this enum was used at other places than as a property of the `@Field` annotation.
diff --git a/src/main/java/org/springframework/data/elasticsearch/annotations/Field.java b/src/main/java/org/springframework/data/elasticsearch/annotations/Field.java
index 3210ce512..84d6b240e 100644
--- a/src/main/java/org/springframework/data/elasticsearch/annotations/Field.java
+++ b/src/main/java/org/springframework/data/elasticsearch/annotations/Field.java
@@ -140,7 +140,7 @@
 	/**
 	 * @since 4.0
 	 */
-	Similarity similarity() default Similarity.Default;
+	String similarity() default Similarity.Default;
 
 	/**
 	 * @since 4.0
diff --git a/src/main/java/org/springframework/data/elasticsearch/annotations/InnerField.java b/src/main/java/org/springframework/data/elasticsearch/annotations/InnerField.java
index 1a7e14ecf..3d1bd46e6 100644
--- a/src/main/java/org/springframework/data/elasticsearch/annotations/InnerField.java
+++ b/src/main/java/org/springframework/data/elasticsearch/annotations/InnerField.java
@@ -109,7 +109,7 @@
 	/**
 	 * @since 4.0
 	 */
-	Similarity similarity() default Similarity.Default;
+	String similarity() default Similarity.Default;
 
 	/**
 	 * @since 4.0
diff --git a/src/main/java/org/springframework/data/elasticsearch/annotations/Similarity.java b/src/main/java/org/springframework/data/elasticsearch/annotations/Similarity.java
index 8761cf29b..dd1ce02a6 100644
--- a/src/main/java/org/springframework/data/elasticsearch/annotations/Similarity.java
+++ b/src/main/java/org/springframework/data/elasticsearch/annotations/Similarity.java
@@ -19,18 +19,9 @@
  * @author Peter-Josef Meisch
  * @since 4.0
  */
-public enum Similarity {
-	Default("default"), BM25("BM25"), classic("classic"), Boolean("boolean");
-
-	// need to use a custom name because 'boolean' can't be used as enum name
-	private final String toStringName;
-
-	Similarity(String name) {
-		this.toStringName = name;
-	}
-
-	@Override
-	public String toString() {
-		return toStringName;
-	}
+public final class Similarity {
+	public final static String Default = "default";
+	public final static String BM25 = "BM25";
+	public final static String classic = "classic";
+	public final static String Boolean = "boolean";
 }
diff --git a/src/main/java/org/springframework/data/elasticsearch/core/index/MappingParameters.java b/src/main/java/org/springframework/data/elasticsearch/core/index/MappingParameters.java
index 1489bb9d2..fcd15ebe0 100644
--- a/src/main/java/org/springframework/data/elasticsearch/core/index/MappingParameters.java
+++ b/src/main/java/org/springframework/data/elasticsearch/core/index/MappingParameters.java
@@ -110,7 +110,7 @@ public final class MappingParameters {
 	private final Integer dims;
 	private final String searchAnalyzer;
 	private final double scalingFactor;
-	private final Similarity similarity;
+	private final String similarity;
 	private final boolean store;
 	private final TermVector termVector;
 	private final FieldType type;
@@ -330,8 +330,8 @@ public void writeTypeAndParametersTo(ObjectNode objectNode) throws IOException {
 			objectNode.put(FIELD_PARAM_POSITION_INCREMENT_GAP, positionIncrementGap);
 		}
 
-		if (similarity != Similarity.Default) {
-			objectNode.put(FIELD_PARAM_SIMILARITY, similarity.toString());
+		if (!Similarity.Default.equals(similarity)) {
+			objectNode.put(FIELD_PARAM_SIMILARITY, similarity);
 		}
 
 		if (termVector != TermVector.none) {