Gas–liquid chromatography (GLC) has been applied to the study of thermodynamic interactions in poly(vinyl chloride) (PVC) plasticized by di-n-octyl phthalate (DnOP). A number of vapor-phase “probes” were used to evaluate the Flory-Huggins thermodynamic parameter for the PVC–DnOP interaction in stationary phase mixtures of the components which covered the entire composition range. Experiments were carried out in the temperature span of 100–130°C. The interaction parameter was strongly negative, indicating high PVC–DnOP compatibility, up to 0.25 volume fraction of plasticizer. It then became less negative and finally positive at 0.55 volume fraction of DnOP, suggesting a lower compatibility limit. The composition dependence of the interaction parameter and its apparent variation with the chemical nature of the vapor-phase probe may reflect a nonrandom solution of the probe in the stationary phase and/or non-random mixing of PVC–DnOP, particularly at DnOP contents in the limited compatibility range. Evaluations of the influence of DnOP on zero-shear melt viscosity and Tg of compounds indicate that both thermodynamic interactions and volume-of-dilution effects must be taken into account in assessing the effectiveness of the plasticizer.