The two heteropolyacids H(3)PW(12)O(40) and H(3)PMo(12)O(40), and their homologous salts (NH(4))(3)PW(12)O(40), and (NH(4))(3)PMo(12)O(40) were prepared and tested in the cationic polymerization of styrene. The results showed that the heteropolytungstates were more reactive than the heteropolymolybdates. It has been found that the yield and the viscosity average molecular weight (M(v)) of polystyrene are directly proportional to the acidity strength of the heteropolyanions (H(3)PW(12)O(40) > H(3)PMo(12)O(40) > (NH(4))(3)PW(12)O(40) > (NH(4))(3)PMo(12)O(40)). The highest yield (68.0%) and M(v) (7,930) were obtained by using H(3)PW(12)O(40). In addition, H(3)PW(12)O(40) polymerized the styrene under mild conditions and was recyclable, and could behave as a truly heterogeneous catalyst.