Enthalpy relaxation at temperatures below the glass transition has been measured for three substituted polystyrene samples and for one poly(alpha-methylstyrene) sample. The overall changes in enthalpy between the experimental glass and the fully relaxed glass (Delta H-infinity) were estimated by fitting experimental data to the three-parameter Cowie-Ferguson expression. These were compared with enthalpy relaxation in polystyrene itself. There was no experimentally detectable effect from the para substituent on the progress of enthalpy loss in the samples. Poly(4-hydroxystyrene), however, was found to exhibit unexpectedly high values of Delta H-infinity. The utility of the Cowie-Ferguson approach with respect to multiparameter phenomenological kinetic expressions for enthalpy relaxation is discussed.