The glass-transition temperatures (T,'s) and specific heats (C,,) of poly(viny1 chloride) (PVC) and PVC plasticized with 5-120 phr di(2-ethylhexyl) adipate (DOA) and tri(2-ethylhexyl) trimellitate (TOTM) have been determined by differential scanning calorimetry (DSC). Measured Tg's were compared to predictions by the Couchman and Karasz (C-K) thermodynamic theory, three related empirical equations, and a new equation obtained from the C-K relation by assuming the product T,AC, to be constant. It was found that the Tg's of the PVC/TOTM mixtures are adequately predicted only by the C-K and the derivative relation. The T,'s of the PVC/DOA mixtures follow a sigmoidal or cusp-like dependence on plasticizer composition as has been observed for some other PVC/plasticizer mixtures. In this case, the approximation afforded by the C-K or derivative equations is still superior to the empirical models over a wide composition range. Dynamic mechanical analysis of the PVC/DOA mixtures suggests that the DSC transitions may consist of two overlapping phase transitions. The reported sigmoidal composition dependence of the DSC Tg 's may therefore result from the measured Tg's being weighted towards the temperature corresponding to the predominant dynamic mechanical transition (i.e., the high T, phase at low plasticizer concentrations and the low Tg phase at high plasticizer concentrations). In such cases of partial phase separation, the C-K or the derivative equation may be used to estimate the composition of the two phases a t each overall plasticizer concentration.