The structural relaxation process in polystyrene and three polystyrene derivatives (p-methyl, p-chloro, and p-hydroxy) at temperatures close to the glass transition temperature has been monitored by differential calorimetric measurements of the heat capacities of the polymers after they were subjected to several different thermal histories. The heat capacity (c(p)) data were analyzed in terms of the recent configurational entropy model proposed by Gomez and Monleon, and good agreement was found between the experimental and model-simulated DSC thermograms. The effect. of the three para substituents upon the structural relaxation process is also discussed.