The long-term thermal stability of poled polymers is of crucial importance because the poled structure is inherently in a non-equilibrium state and relaxation in the poled polymers results from thermal reorientation effects. A quantitative understanding of the relaxation behavior is therefore of great use in optimizing the design of new polymer systems. The major factors involved are explained, and various theoretical models are applied to some nonlinear optical polymers developed at Hoechst.