Methods for chemical modification of polyvinyl chloride and poly(p-chloromethylstyrene) with thiosalicylic acid using the synthesis-in-gel approach were developed. The maximal degree of functionalization of the products reaches 70 and 74% in the case of polyvinyl chloride and poly(p-chloromethylstyrene), respectively, when using a twofold molar excess of thiosalicylic acid in the presence of a base. The composition of the products was determined by elemental analysis, and their structural features were examined by Fourier IR spectroscopy and thermal gravimetric analysis in combination with synchronous IR analysis of the degradation products. Thiosalicylic acid exhibits both S- and O-nucleophilicity in reactions with polyvinyl chloride and poly(p-chloromethylstyrene). The materials prepared from the polyvinyl chloride and poly(p-chloromethylstyrene) derivatives demonstrated high ability to take up ions of the Irving-Williams series.