Thermal decomposition and glass-transition temperature studies have been carried out on poly-p-methylstyrene (PpMeS) with the DSC-2 differential scanning calorimeter. The undecomposed and decomposed polymers were analyzed by gel-permeation chromatography (GPC), infrared spectroscopy (IR), and nuclear magnetic resonance (NMR). Decompositions of PpMeS under isothermal conditions during 50-min periods in the 25–330°C temperature range did not affect Mwvalues; however, Mnvalues decreased from 5.1 × 104to 1.5 × 104. The decomposed polymers following treatments at temperatures above 330°C are only partially soluble in THF, o-C6H4Cl2, and CCl4. These products may therefore contain certain crosslinked materials. The results of the IR analyses show that in the polymers decomposed at temperatures above 330°C, the aromatic content is lower than that observed for polymers decomposed in the 25–330°C temperature range. The variation of Tge(at q = l°K/min) with Mnobeys the relation; Tge(K) = 384–2.65 × 105/Mn. The value of 384° K for the Tgof PpMeS at infinite molecular weight is close to that found for PS (379°K) but is far below that (453°K) obtained for PαMeS. This illustrates the relative importance of the methyl substitution in the vinyl chain as compared to that in the aromatic rings.