The application of a Simon-type equation to the relationship between glass transition temperature and pressure for polymers is described. An equation of this form can be derived if it is assumed that one of the Ehrenfest relationships is valid and that the ratio DELTAC(p)/(V(g)DELTAalpha) depends linearly on pressure. Data obtained by high-pressure DTA for polystyrene were analyzed and it was found that the Simon equation provided a better fit than did a quadratic equation.