The catalytic activity of the iridium complexes involved in methanol carbonylation is significantly enhanced when the carbonyliodoplatinum dimer [PtI2(CO)]2 (10') is added to the reaction mixt. Under CO this complex readily affords the monomeric species [PtI2(CO)2] (10). The turnover frequency value, which is 1450 h-1 for iridium alone, reaches 2400 h-1 for a Pt/Ir = 3/7 molar ratio, under 30 bar of CO and at 190°C. To get a deeper insight into the role of the platinum cocatalyst, model conditions (dinitrogen, ambient temp., CH2Cl2, PPN+ as counterion) have been adopted. [PtI2(CO)]2 (10') interacts with [PPN][IrI3(CH3)(CO)2] (4-PPN), affording the monoiodo-bridged anionic species [IrI2(CH3)(CO)2(μ-I)PtI2(CO)]- (11), which undergoes cleavage under CO to provide [IrI2(CH3)(CO)3] (6) and [PtI3(CO)]- (9). Although we have to take into account the possible iodide dissocn. from 4 in the polar reaction medium (CH3COOH, CH3OH, CH3I, HI, H2O), which can be scavenged by platinum to give 9, we should not discard the intermediacy of 11, even under working catalytic conditions. The crystal structures of [PPN][IrI3(COCH3)(CO)2] (8-PPN) and [PPN][PtI3(CO)] (9-PPN), which are both involved in the overall process, were detd. by X-ray diffraction anal. A catalytic cycle is herein proposed, in which the cooperative effect between the platinum promoter and the iridium catalyst is depicted.