12-Molybdophosphoric acid (12-MPA) was studied in present paper as a special polymeried catalyst with a dual catalytic role of initiation and inhibition. Dilatometry results indicated that 12-MPA played its inhibition function in free radical polymerization of styrene initiated by AIBN, although we had reported the initiation role not long ago. 12-MPA inhibition mechanism was put forward by ESR spectrum as redox reaction between additive of 12-MPA and initiator of AIBN, which was supported by IR, TG/DTA and GC-MS studies. Thus, the dual role of 12-MPA in styrene polymerization was put forward, that is, 12-MPA played as cationic polymerization initiator when it alone in styrene, but as inhibitor when existed together with AIBN. In the condition of the latter, an excessive problem was discussed, e.g. if part of AIBN remained after oxidation, styrene would be polymerized following free radical mechanism, but the reaction rate was slower than that without 12-MPA (12-MPA was expressed as inhibitor); on the contrary, if the residual reagent was 12-MPA after redox, styrene polymerization would still take place, but styrene would be polymerized following cationic mechanism. (C) 2003 Elsevier B.V. All rights reserved.