Solid-get transition temperatures, t(m), were measured for PVC in binary and ternary mixtures of plasticizers. Data were used to investigate interactions between plasticizers, and their effect on t(m) and polymer-plasticizer interaction (chi) values. The UNIFAC-FV method was used to predict Gibbs free energy of mixing, and chi for a range of plasticizers, and interactions for one quaternary mixture. Synergy was observed when one or more branched phthalates were mixed with ODPP, but none when three phthalates were mixed. Mixtures of DOP with aliphatic ester plasticizers again produced synergy. Equations were produced to enable interaction coefficients of different plasticizer blends to be predicted. It was found that interaction coefficients increase as the difference in molar mass between the two plasticizers increases. Calculated chi values were generally similar to experimental values. The lower limit of Gibbs free energy of mixing required for plasticization of PVC using monomeric plasticizers was calculated to be about -0.9 J/g, and that for polymeric plasticizers -0.15 J/g. Equations were produced enabling Gibbs free energy of mixing to be predicted from the amount of C atoms in the plasticizer. (C) 2003 Elsevier Ltd. All rights reserved.