Glass transition temperature (Tg) studies on poly-p-isopropyl-α-methylstyrene havingMnvalues ranging from 5 × 103to 1.53 × 105were carried out with the DSC-2 differential scanning calorimeter. The variation of Tge(at q = l°K/min) withMnobeys the relation, Tge(°K) = 422–3.6 × 105/M. A comparison of Tg∞values for polystyrene (379°K), poly-α-methylstyrene (453°K), and poly-p-isopropyl-α-methylstyrene (422°K) reveals that although the addition of an α-methyl group in polystyrene increases its Tg∞from 379 to 453°K due to steric hindrance, the addition of the p-isopropyl group in poly-α-methylstyrene lowers it from 453 to 422°K due to reduced chain stiffness and smaller interchain steric hindrance. Poly-p-isopropyl-α-methylstyrene samples having similarMnbut different Tgvalues are shown to possess different structures. The effect of polydispersity on Tgis also stressed.