Copolymers based on substituted poly(vinyl chloride) (PVC) produced by reacting PVC with different thiolates in cyclohexanone solution and in the absence of solvent (melt), show different glass transition temperatures, T-8, for the same chemical composition. The variation of T-g for the vinylchloride-vinylthiobenzene, vinylchloride-vinylthiocresol and vinylchloride-vinylthionaphthol copolymers obtained in the melt follows the Fox equation. However, the same copolymers obtained in cyclohexanone show a deviation from this trend of T-g as a function of molar composition. Factors such as microstructure, chemical composition distribution and molecular weight do not change appreciably from solution to the melt, and therefore cannot be responsible for these differences in the T-g values. We attribute the deviation in the T-g values of copolymers obtained in cyclohexanone solution to polymer-polymer interactions produced at local sites on the chain, and we propose that these interactions are related to chlorine atoms in isotactic and/or heterotactic triads.