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.