The change of the polymorphic behavior of syndiotactic polystyrene after introduction of different amounts (3-20 mol %) of p-methylstyrene comonomeric units is presented. Variations of the polymorphic behavior have been observed for solvent extraction procedures, for annealing processes,and for solution and melt crystallizations. The behavior of samples subjected to extraction and annealing procedures indicates that the introduction of p-methylstyrene comonomeric units tends to destabilize the clathrate delta-form. For the solution as well as for the melt crystallization, the increase of p-methylstyrene content favors obtaining the alpha-form over the beta-form and, in particular, for the 20 mol % p-methylstyrene copolymer, the beta-form is never obtained. Both kinetic and thermodynamic factors seem to be involved. Structural variations for the alpha-form with the introduction of p-methylstyrene units are also investigated.