The mercuration of soluble polystyrene was investigated, and it was found that an addition of perchoric acid to the reaction mixture significantly accelerated the polymer transformation. The mercuration proceeds as a polymer analogous reaction, i.e., without any change in the degree of the polymerization. The mercurated product could be converted back to the original polystyrene by hydrolysis or to the corresponding bromopolystyrene by bromination. Ultracentrifuge measurements show that even at low conversion rates all macromolecules were mercurated and unreacted polystyrene could not be identified in the product. Calorimetry indicated the presence of styrene–mercuric acetate blocks in copolymers with high mercury contents.