The nucleophilic substitution reaction on poly(vinyl chloride) (PVC) with sodium 2-mercaptobenzothiazolate has been carried out in solution in cyclohexanone at different temperatures. The kinetic parameters indicate the occurrence of one process having an activation energy of 63 KJ.mol(-1). The evolution of unreacted iso, hetero and syndiotactic triad contents with degree of substitution has been followed by C-13 n.m.r. spectroscopy. By comparing quantitatively the microstructural changes with degree of substitution and taking into account that the reaction is of S(N)2 type only the mechanisms of substitution through the mm triad of mmr tetrad and the rm of rrmr pentads are proved to react. On the contrary, owing to steric hindrance the reaction by the mr triad in the mmr tetrad is quite unlikely, thus enhancing the stereoselective nature of the reaction. This conclusion was confirmed on the basis of the FTi.r. results and is more feasible than using other nucleophiles to study the effect of the aforementioned structures on the polymer properties and therefore provides new approaches to better comprehension at the molecular level of the structure-property relationships. These novel contributions are an original approach to the prominent role of the microstructure-dependent local conformation in the mechanisms of analogous reactions of PVC.