Different compounds for the synthesis of poly(vinyl chloride) (PVC) with tertiary amino groups were tested, and the course of the modification reactions was followed under different conditions by nuclear magnetic resonance spectroscopy and elemental analysis. It is shown that PVC can be modified without side reactions with 2-mercaptopyridine, 2-mercaptopyrimidine, 4-mercapto-N,N-dimethylaniline, and 4-mercaptopyridine. The reactivity of the para-substituted mercapto compounds is found to be considerably higher than that of the corresponding ortho products, and higher final degrees of modification are achieved. The availability of the amino group towards electrophilic attack in order to form quaternary ammonium salts was tested by reaction with methyl iodide. While this reaction takes place in good yields in the case of PVC modified with 4-mercaptopyridine and 4-mercapto-N,N-dimethylaniline, aminated PVC with the nitrogen atoms in ortho position do not react due to steric hindrance by the polymer chain.