AbstractTwo‐phase polymeric materials such as polymer blends, block copolymers, and graft copolymers, are thermorheologically complex. Mechanical response curves obtained on such materials at different temperatures cannot, in principle, be brought into superposition by a simple shift along the logarithmic time or frequency axis. The shift factors become functions of time or frequency in addition to temperature.A general treatment of time‐temperature superposition in thermorheologically complex materials is developed and a model is proposed from which, for a two‐phase material, the amount of shift can be calculated which is necessary to bring a point on a mechanical response curve obtained at a given temperature and time or frequency into superposition at another temperature. The mechanical responses of the constituent homopolymers and their temperature functions must be known.