The shift factoraT for thermorheologically complex bodies is a function not only of temperature T but also of the deformation time t. A method is given for determining the vertical shift factor on the basis of macro-tests within the experimental time window. A procedure for determining the shift functionaT(T, t) is described with reference to the isothermal creep of a polyester glass-reinforced plastic in shear in the plane of reinforcement, and a master curve, simulating the long-term creep of a thermorheologically complex composite, is constructed. The predictions are compared with a long-term control experiment.