Data traffic between a mobile radio network and an IMS network is secured by first authenticating a mobile subscriber in the mobile radio network and in the IMS network. Next, an examination is carried out to check whether the identity of the mobile subscriber authenticated in the IMS network corresponds to the identity of the subscriber authenticated in the mobile radio network. If so, a confirmation message is sent from the IMS network to the mobile subscriber in the even of corresponding identities and a data exchange is carried out between the mobile subscriber and the IMS network by a security protocol protected by a common key derived from the confirmation message.