The inverse gas chromatography method was extended to study the concentration dependent Flory-Huggins interaction parameter between poly(vinyl chloride) (PVC) and plasticizers using literature data. For both PVC/polyadipate and PVC/epoxidized soybean oil (ESO) systems, the miscibility was better at the high PVC end. The specific interaction between PVC and plasticizers was estimated from the difference between the experimental results and the enthalpies of mixing predicted by the solubility parameter model. The interaction was negative and skewed toward the high PVC end, and rendered the overall interaction parameter negative at high PVC compositions. Chemical potential was used to determine the phase composition of PVC/plasticizer mixtures. (C) 2003 Wiley Periodicals, Inc.