A viscoelastic body which undergoes a thermomechanical process is considered. The evolution of the state variables, i.e., the displacement and temperature fields, is described by a coupled hyperbolic–parabolic integrodifferential system. The heat flux through a portion of the boundary is controlled automatically by a thermostat device acting on the external temperature according to suitable measurements of the boundary strain and/or the boundary temperature. Two types of control are analysed: relay switch and Preisach hysteresis operator. In both cases, the resulting model is an initial and boundary value problem for the evolution system with non‐linear and non‐local boundary conditions for the temperature. Existence and uniqueness results are proved. Copyright © 1999 John Wiley & Sons, Ltd.