diff --git a/pkg/mock/handler.go b/pkg/mock/handler.go
index feb8a26..4d98570 100644
--- a/pkg/mock/handler.go
+++ b/pkg/mock/handler.go
@@ -188,7 +188,11 @@ func (ip *identityProviderHandler) Token(w http.ResponseWriter, r *http.Request)
 	idToken.Set("acr", auth.AcrLevel)
 	idToken.Set("iat", time.Now().Unix())
 	idToken.Set("exp", time.Now().Unix()+expires)
-	idToken.Set("sid", sid)
+	if !ip.Provider.OpenIDConfiguration.GetCheckSessionIframe() {
+		idToken.Set("sid", sid)
+	} else {
+		idToken.Set("session_state", sid)
+	}
 
 	signedIdToken, err := ip.signToken(idToken)
 	if err != nil {
diff --git a/pkg/mock/openid.go b/pkg/mock/openid.go
index c8e6409..cd1fa4c 100644
--- a/pkg/mock/openid.go
+++ b/pkg/mock/openid.go
@@ -6,8 +6,7 @@ import (
 	"github.com/go-chi/chi/v5"
 )
 
-func IdentityProviderServer() (*httptest.Server, TestProvider) {
-	provider := NewTestProvider()
+func IdentityProviderServer(provider TestProvider) (*httptest.Server, TestProvider) {
 	handler := newIdentityProviderHandler(provider)
 	router := identityProviderRouter(handler)
 	server := httptest.NewServer(router)
diff --git a/pkg/mock/provider.go b/pkg/mock/provider.go
index 0971a19..716817c 100644
--- a/pkg/mock/provider.go
+++ b/pkg/mock/provider.go
@@ -31,6 +31,11 @@ func (p TestProvider) PrivateJwkSet() *jwk.Set {
 	return &p.JwksPair.Private
 }
 
+func (p TestProvider) WithCheckSessionIframe() TestProvider {
+	p.OpenIDConfiguration.CheckSessionIframe = "https://some-url/checksession"
+	return p
+}
+
 func NewTestProvider() TestProvider {
 	jwksPair, err := crypto.NewJwkSet()
 	if err != nil {
diff --git a/pkg/openid/configuration.go b/pkg/openid/configuration.go
index a8e11cc..34bf7ff 100644
--- a/pkg/openid/configuration.go
+++ b/pkg/openid/configuration.go
@@ -73,12 +73,8 @@ func (c *Configuration) FetchJwkSet(ctx context.Context) (*jwk.Set, error) {
 	return &jwkSet, nil
 }
 
-func (c *Configuration) FetchCheckSessionIframe() bool {
-	if c.CheckSessionIframe == "" {
-		return false
-	}
-
-	return true
+func (c *Configuration) GetCheckSessionIframe() bool {
+	return c.CheckSessionIframe != ""
 }
 
 func (c *Configuration) SidClaimRequired() bool {
diff --git a/pkg/router/handler_callback.go b/pkg/router/handler_callback.go
index 35e2346..6eb43e2 100644
--- a/pkg/router/handler_callback.go
+++ b/pkg/router/handler_callback.go
@@ -123,7 +123,7 @@ func (h *Handler) ExternalSessionId(idToken *openid.IDToken) (string, error) {
 	switch {
 	case openIDconfig.SidClaimRequired():
 		externalSessionID, err = idToken.GetStringClaim("sid")
-	case openIDconfig.FetchCheckSessionIframe():
+	case openIDconfig.GetCheckSessionIframe():
 		externalSessionID, err = idToken.GetStringClaim("session_state")
 	default:
 		externalSessionID = h.GenerateExternalSessionID()
diff --git a/pkg/router/router_test.go b/pkg/router/router_test.go
index e923d6a..24db283 100644
--- a/pkg/router/router_test.go
+++ b/pkg/router/router_test.go
@@ -44,7 +44,7 @@ func newHandler(provider openid.Provider) *router.Handler {
 }
 
 func TestHandler_Login(t *testing.T) {
-	idpserver, idp := mock.IdentityProviderServer()
+	idpserver, idp := mock.IdentityProviderServer(mock.NewTestProvider())
 	h := newHandler(idp)
 	r := router.New(h)
 
@@ -100,7 +100,7 @@ func TestHandler_Login(t *testing.T) {
 }
 
 func TestHandler_Callback_and_Logout(t *testing.T) {
-	idpserver, idp := mock.IdentityProviderServer()
+	idpserver, idp := mock.IdentityProviderServer(mock.NewTestProvider())
 
 	h := newHandler(idp)
 	r := router.New(h)
@@ -195,7 +195,80 @@ func TestHandler_Callback_and_Logout(t *testing.T) {
 }
 
 func TestHandler_FrontChannelLogout(t *testing.T) {
-	_, idp := mock.IdentityProviderServer()
+	_, idp := mock.IdentityProviderServer(mock.NewTestProvider())
+	h := newHandler(idp)
+	r := router.New(h)
+	server := httptest.NewServer(r)
+
+	idp.ClientConfiguration.RedirectURI = server.URL + "/oauth2/callback"
+	idp.ClientConfiguration.PostLogoutRedirectURI = server.URL
+
+	jar, err := cookiejar.New(nil)
+	assert.NoError(t, err)
+
+	client := server.Client()
+	client.Jar = jar
+	client.CheckRedirect = func(req *http.Request, via []*http.Request) error {
+		return http.ErrUseLastResponse
+	}
+
+	// First, run /oauth2/login to set cookies
+	resp, err := client.Get(server.URL + "/oauth2/login")
+	assert.NoError(t, err)
+	assert.Equal(t, http.StatusTemporaryRedirect, resp.StatusCode)
+	defer resp.Body.Close()
+
+	// Get authorization URL
+	location := resp.Header.Get("location")
+	u, err := url.Parse(location)
+	assert.NoError(t, err)
+
+	// Follow redirect to authorize with idporten
+	resp, err = client.Get(u.String())
+	assert.NoError(t, err)
+	assert.Equal(t, http.StatusTemporaryRedirect, resp.StatusCode)
+	defer resp.Body.Close()
+
+	// Get callback URL after successful auth
+	location = resp.Header.Get("location")
+	callbackURL, err := url.Parse(location)
+	assert.NoError(t, err)
+
+	// Follow redirect to callback
+	resp, err = client.Get(callbackURL.String())
+	assert.NoError(t, err)
+	assert.Equal(t, http.StatusTemporaryRedirect, resp.StatusCode)
+
+	cookies := client.Jar.Cookies(callbackURL)
+	sessionCookie := getCookieFromJar(router.SessionCookieName, cookies)
+
+	assert.NotNil(t, sessionCookie)
+
+	// Trigger front-channel logout
+	ciphertext, err := base64.StdEncoding.DecodeString(sessionCookie.Value)
+	assert.NoError(t, err)
+
+	sid, err := h.Crypter.Decrypt(ciphertext)
+	assert.NoError(t, err)
+
+	frontchannelLogoutURL, err := url.Parse(server.URL)
+	assert.NoError(t, err)
+
+	frontchannelLogoutURL.Path = "/oauth2/logout/frontchannel"
+
+	values := url.Values{}
+	values.Add("sid", string(sid))
+	values.Add("iss", idp.GetOpenIDConfiguration().Issuer)
+	frontchannelLogoutURL.RawQuery = values.Encode()
+
+	resp, err = client.Get(frontchannelLogoutURL.String())
+	assert.NoError(t, err)
+	assert.Equal(t, http.StatusOK, resp.StatusCode)
+	defer resp.Body.Close()
+}
+
+func TestHandler_FrontChannelLogoutWithCheckSessionIframe(t *testing.T) {
+	_, idp := mock.IdentityProviderServer(mock.NewTestProvider().WithCheckSessionIframe())
 	h := newHandler(idp)
 	r := router.New(h)
 	server := httptest.NewServer(r)