diff --git a/README.md b/README.md
index 50d3a2e..75c8c7e 100644
--- a/README.md
+++ b/README.md
@@ -392,7 +392,11 @@ specifies the element type. For maps, the tag has the format
 <a id="markdown-custom-decoders-mappers" name="custom-decoders-mappers"></a>
 ## Custom decoders (mappers)
 
-If a field implements the [MapperValue](https://godoc.org/github.com/alecthomas/kong#MapperValue)
+Any field implementing `encoding.TextUnmarshaler` or `json.Unmarshaler` will use those interfaces
+for decoding values.
+
+For more fine-grained control, if a field implements the
+[MapperValue](https://godoc.org/github.com/alecthomas/kong#MapperValue)
 interface it will be used to decode arguments into the field.
 
 <a id="markdown-supported-tags" name="supported-tags"></a>
diff --git a/go.mod b/go.mod
index c6f418f..8bbc12d 100644
--- a/go.mod
+++ b/go.mod
@@ -2,7 +2,6 @@ module github.com/alecthomas/kong
 
 require (
 	github.com/davecgh/go-spew v1.1.1 // indirect
-	github.com/mitchellh/mapstructure v1.1.2
 	github.com/pkg/errors v0.8.1
 	github.com/pmezard/go-difflib v1.0.0 // indirect
 	github.com/stretchr/testify v1.2.2
diff --git a/go.sum b/go.sum
index 22a1f3c..c935d68 100644
--- a/go.sum
+++ b/go.sum
@@ -1,7 +1,5 @@
 github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
-github.com/mitchellh/mapstructure v1.1.2 h1:fmNYVwqnSfB9mZU6OS2O6GsXM+wcskZDuKQzvN1EDeE=
-github.com/mitchellh/mapstructure v1.1.2/go.mod h1:FVVH3fgwuzCH5S8UJGiWEs2h04kUh9fWfEaFds41c1Y=
 github.com/pkg/errors v0.8.1 h1:iURUrRGxPUNPdy5/HRSm+Yj6okJ6UtLINN0Q9M4+h3I=
 github.com/pkg/errors v0.8.1/go.mod h1:bwawxfHBFNV+L2hUp1rHADufV3IMtnDRdf1r5NINEl0=
 github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
diff --git a/mapper_test.go b/mapper_test.go
index 12def43..3338cff 100644
--- a/mapper_test.go
+++ b/mapper_test.go
@@ -2,11 +2,13 @@ package kong_test
 
 import (
 	"bytes"
+	"encoding/json"
 	"fmt"
 	"io/ioutil"
 	"net/url"
 	"os"
 	"reflect"
+	"strings"
 	"testing"
 	"time"
 
@@ -28,6 +30,45 @@ func TestValueMapper(t *testing.T) {
 	require.Equal(t, "MOO", cli.Flag)
 }
 
+type textUnmarshalerValue string
+
+func (m *textUnmarshalerValue) UnmarshalText(text []byte) error {
+	*m = textUnmarshalerValue(strings.ToUpper(string(text)))
+	return nil
+}
+
+func TestTextUnmarshaler(t *testing.T) {
+	var cli struct {
+		Value textUnmarshalerValue
+	}
+	p := mustNew(t, &cli)
+	_, err := p.Parse([]string{"--value=hello"})
+	require.NoError(t, err)
+	require.Equal(t, "HELLO", string(cli.Value))
+}
+
+type jsonUnmarshalerValue string
+
+func (j *jsonUnmarshalerValue) UnmarshalJSON(text []byte) error {
+	var v string
+	err := json.Unmarshal(text, &v)
+	if err != nil {
+		return err
+	}
+	*j = jsonUnmarshalerValue(strings.ToUpper(v))
+	return nil
+}
+
+func TestJSONUnmarshaler(t *testing.T) {
+	var cli struct {
+		Value jsonUnmarshalerValue
+	}
+	p := mustNew(t, &cli)
+	_, err := p.Parse([]string{"--value=hello"})
+	require.NoError(t, err)
+	require.Equal(t, "HELLO", string(cli.Value))
+}
+
 func TestNamedMapper(t *testing.T) {
 	var cli struct {
 		Flag string `type:"moo"`