diff --git a/env.go b/env.go
index 1a82ee4..db8665b 100644
--- a/env.go
+++ b/env.go
@@ -327,6 +327,13 @@ func Marshal(v interface{}) (EnvSet, error) {
 		}
 
 		for _, envKey := range envKeys {
+			// Skip keys with '=', as they represent tag options and not environment variable names.
+			if strings.Contains(envKey, "=") {
+				switch strings.ToLower(strings.SplitN(envKey, "=", 2)[0]) {
+				case "separator", "required", "default":
+					continue
+				}
+			}
 			es[envKey] = envValue
 		}
 	}
diff --git a/env_test.go b/env_test.go
index d68ee29..70aad30 100644
--- a/env_test.go
+++ b/env_test.go
@@ -61,6 +61,14 @@ type ValidStruct struct {
 
 	MultipleTags string `env:"npm_config_cache,NPM_CONFIG_CACHE"`
 
+	MultipleTagsWithDefault string `env:"multiple_tags_with_default,MULTIPLE_TAGS_WITH_DEFAULT,default=default_tags_value"`
+
+	TagWithDefault string `env:"tag_with_default,default=default_tag_value"`
+
+	TagWithRequired string `env:"tag_with_required,required=false"`
+
+	TagWithSeparator string `env:"tag_with_separator,separator=&"`
+
 	// time.Duration is supported
 	Duration time.Duration `env:"TYPE_DURATION"`
 
@@ -474,14 +482,18 @@ func TestMarshal(t *testing.T) {
 		}{
 			Workspace: "/mnt/builds/slave/workspace/test",
 		},
-		Extra:        "extra",
-		Int:          1,
-		Uint:         4294967295,
-		Float32:      float32(2.3),
-		Float64:      4.5,
-		Bool:         true,
-		MultipleTags: "foobar",
-		Duration:     3 * time.Minute,
+		Extra:                   "extra",
+		Int:                     1,
+		Uint:                    4294967295,
+		Float32:                 float32(2.3),
+		Float64:                 4.5,
+		Bool:                    true,
+		MultipleTags:            "foobar",
+		MultipleTagsWithDefault: "baz",
+		TagWithDefault:          "bar",
+		TagWithRequired:         "foo",
+		TagWithSeparator:        "val1&val2",
+		Duration:                3 * time.Minute,
 	}
 
 	environ, err := Marshal(&validStruct)
@@ -529,6 +541,38 @@ func TestMarshal(t *testing.T) {
 		t.Errorf("Expected field value to be '%s' but got '%s'", "foobar", environ["NPM_CONFIG_CACHE"])
 	}
 
+	if environ["multiple_tags_with_default"] != "baz" {
+		t.Errorf("Expected field value to be '%s' but got '%s'", "baz", environ["multiple_tags_with_default"])
+	}
+
+	if environ["default=default_tags_value"] != "" {
+		t.Errorf("'default=default_tags_value' not expected to be a valid field value.")
+	}
+
+	if environ["tag_with_default"] != "bar" {
+		t.Errorf("Expected field value to be '%s' but got '%s'", "bar", environ["tag_with_default"])
+	}
+
+	if environ["tag_with_required"] != "foo" {
+		t.Errorf("Expected field value to be '%s' but got '%s'", "foo", environ["tag_with_required"])
+	}
+
+	if environ["tag_with_separator"] != "val1&val2" {
+		t.Errorf("Expected field value to be '%s' but got '%s'", "val1&val2", environ["tag_with_separator"])
+	}
+
+	if environ["required=true"] != "" {
+		t.Errorf("'required=true' not expected to be a valid field value.")
+	}
+
+	if environ["separator=&"] != "" {
+		t.Errorf("'separator=&' not expected to be a valid field value.")
+	}
+
+	if environ["default=default_tag_value"] != "" {
+		t.Errorf("'default=default_tag_value' not expected to be a valid field value.")
+	}
+
 	if environ["TYPE_DURATION"] != "3m0s" {
 		t.Errorf("Expected field value to be '%s' but got '%s'", "3m0s", environ["TYPE_DURATION"])
 	}