The enthalpy of mixing (DELTA(M)H) for polystyrene with poly(alpha-methylstyrene) has been estimated by a semidirect calorimetric method. This involved measuring heats of solution of blends and of mechanical mixtures having the same composition to obtain, by difference, the heat of mixing of the two polymers in their glassy states. By assuming a model for the relaxation of a glass to its corresponding hypothetical liquid state, these were then converted to give a DELTA(M)H value for the liquid polymers. A small negative value for the heat of mixing was obtained over part of the composition range, contrary to expectations.