Nucleophilic substitution in poly(vinyl chloride) (PVC) with sodium thiobenzoate (NaTBZ) has been studied in two kinds of solvent. From the evolution of the content of isotactic (mr) and heterotactic (mr) triads in the unmodified parts of the polymer, as investigated by C-13 n.m.r., the reaction was found to behave similarly to that previously studied with sodium benzenethiolate (NaBT) in that only the local configurations mmr and rrmr at the end of isotactic and syndiotactic sequences respectively are reactive. The same reaction with both nucleophiles has also been studied in aqueous suspension in the presence of a phase transfer catalyst. In all cases the ratio of mmr content to rrmr content is found to vary according to the experimental conditions, in particular the type of solvent, irrespective of the nucleophile. Accordingly, model polymers having well known tacticity-derived microstructure can be obtained. On the other hand, the evolution of T-g with degree of conversion has been studied by calorimetric measurements. T-g has been found to change linearly with conversion and the correlation between the corresponding rates and the evolution of the ratio between the mmr and rrmr contents proves to be linear too. From these studies it follows that T-g depends on two well defined factors : one, variable in character, is the nucleophile nature; the other is the microstructure in terms of the isotactic and syndiotactic sequences and, especially, the local configurations and conformations at the end of them. Interestingly, this effect is permanent in character and is independent of the substituent. The results that we have obtained provide new concepts on the involvement of some specific molecular microstructural features in the T-g phenomenon.