The substitution reaction of PVC with sodium dimethyldrthiocarbamate (NaDC) and sodium azide (NaN3) was performed in cyclohexanone and dimel hylformamide at various temperatures. The degree of compositional heterogeneity mas estimated and found to be important up to conversions of about 0,5%, but to vanish afterwards. The evolution of unreacted iso, hetero and syndiotactic triads was measured by I3C NMR spectroscopy. For conversions of at least up to 12%, the content of both the isotactic and heterotactic triads was found to decrease, relative to the syndiotactic ones, but at different relative rates depending on the nucleophile. From these results, together with the kinetic ones, it is concluded that the reaction proceeds by a stereoselective mechanism. The results are discussed on the basis of a hypothesis concerning the role of the local triad conformations in PVC that contain the T T isotactic diad conformation. The results enable the selective substitution of PVC with NaDC and NaN,, which may be used as a useful tool to prepare specific graft copolymers based on PVC.