The glass-transition temperature of ethylene/propylene statistical copolymers is evaluated by using the pseudostereochemical equilibrium approach; i.e., the partition function of a copolymer chain with interactions between neighboring conformations is formally obtained from a fictitious homopolymer wherein the two comonomer units are in equilibrium, with proper conformational constraints to produce the existing amounts of unit pairs. The resulting formalism enables us to avoid the difficult task of evaluating the geometrical average of the partition functions of different chains. The Gibbs-DiMarzio theory is extended to off-lattice models with any stereochemical structure; the rotational-isomeric-state entropy is supplemented with the contribution from fluctuations of the bond rotation angles around their energy minima. Allowing for rotational fluctuations in the glassy state enables one to account for a larger-than-crystalline entropy through the rms fluctuation angle, treated as an adjustable parameter. Although additional experimental data are awaited, the observed trend of T-g vs composition for propylene-rich copolymers is correctly reproduced. It is predicted that copolymers tending to alternation (r(E)r(p) similar to 0) should have a T-g lower by more than 20 degrees C than those with unit blocks (r(E)r(P) > 1), in qualitative agreement with what is observed for vinylidene fluoride/hexafluoropropene copolymers.