The nucleophilic substitution reactions on poly(vinyl chloride) (PVC) with sodium benzenethiolate, sodium thiobenzoate and sodium 2-mercaptobenzothiazolate have been carried out in solution of cyclohexanone. The aim was to study from a comparative point of view the influence of nucleophile characteristics, in particular bulkiness, on the reaction mechanisms through the microstructural evolution with degree of substitution of the same PVC sample with the three kinds of nucleophile in the same experimental conditions. The quantitative microstructural analysis with conversion has been followed by 13C NMR spectroscopy. Owing to steric hindrance of 2-mercaptobenzenethiolate the stereoselective nature of the reaction is enhanced making the reaction through the mr triads of mmr inoperative in practice unlike the other nucleophiles. This conclusion was confirmed on the basis of the FTIR results.