Abstract The technique of inverse gas chromatography was applied to study the miscibility (compatibility) of the system of PVC with epoxidized soybean oil (ESO). The PVC-ESO thermodynamic interaction parameters, χ + 23, were determined from the measured retention data for various vapour-phase “probes” in stationary phase mixtures of the components covering the entire composition range. Experiments were run in the range 90–110°C. In accordance with previous studies on polymer-polymer and polymer-plasticizer blends, the interaction parameters were found to be dependent on the temperature, the chemical nature of the probe and the blend composition. χ + 23 values were mostly negative or slightly positive up to 0.40 weight fraction of ESO, indicating relatively high compatibility of PVC and ESO for this range of compositions. They then increased with increasing plasticizer content and, at ca 0.70 weight fraction of ESO, exceeded 0.5 which is considered as the “compatibility limit value”.