The calorimetric glass‐transition temperature (Tg) and transition width were measured over the full composition range for solvent–solvent mixtures of o‐terphenyl with tricresyl phosphate and with dibutyl phthalate and for polymer–solvent mixtures of polystyrene with three dialkyl phthalates. Tg shifted smoothly to higher temperatures with the addition of the component with the higher Tg for both sets of solvent–solvent mixtures. The superposition of the differential scanning calorimetry traces showed almost no composition dependence for the width of the transition region. In contrast, the composition dependence of Tg in polymer–solvent mixtures was different at high and low polymer concentrations, and two distinct Tg's were observed at intermediate compositions. These results were interpreted in terms of the local length scale and associated local composition variations affecting Tg. The possible implications of these results for the dynamics of miscible polymer blends were examined. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1155–1163, 2004