For propylene/ethylene random copolymers, the relationship between T-g and the composition has been investigated. At a constant composition, T-g is variable depending on the cohesive state of ethylene units in the glass. For polyethylene, the reference values of T-g = 130, 134, 135, 153, and 237 K correspond to the cohesive enthalpies of 3.98, 3.19, 3.23 (=6.46/2), 3.77, and 6.46 kJ per molar ethylene unit in the glasses, respectively. The heat capacity jump at the glass transition for these polymers was simulated using the scheme of melting of ordered parts followed by recrystallization. At the mole fractions of propylene (meso) units of 0.3 and 0.4, a distinct peak is apparent in the heat capacity jump curve.