The composition dependence of the glass transition temperature (T g ) in mixtures remains an important unsolved problem. Here, it is revisited using three model systems: a series of oligomeric and polymeric cyanurates, blends of oligomeric and polymeric α-methyl styrene, and molecular mixtures of itraconazole and posaconazole. We evaluate several entropy-based models to determine the theoretical T g as a function of molecular composition and compare the results against the experimental data. The assumption that the configurational entropy is invariant at the T g is tested, where the change in configurational entropy is assumed to be given by the integral of ΔC p dlnT, where ΔC p is the temperature-dependent change in the heat capacity at T g . We find that, although the temperature-dependent heat capacities in both liquid and glassy states are nearly independent of composition for several of the systems studied (i.e., they are nearly ideal mixtures), the composition dependence of T g is not well described by simply adding the changes in the massweighted configurational entropy of the components on going from the T g in the pure state to that of the blend. The implication is that either configurational entropy is not invariant at T g or that it cannot be obtained from the integral of ΔC p dlnT.