The iodocarbonyl monomer [PtI2(CO)2] 7 promotes the iridium catalyzed carbonylation of methanol to acetic acid at low water contents. Studies based on low pressure or high pressure NMR and the use of labeled reactants were conducted close to the real conditions of catalysis in order to get a deeper insight into this system. Carbonylation of CH3I at low water contents proceeds slowly and the migratory CO insertion step, leading from H[IrI3(CH3)(CO)2] 2-H to H[IrI3(COCH3)(CO)2] 6-H is rate limiting. The dimer [PtI2(CO)]27′ reacts immediately with [PPN][IrI3(CH3)(CO)2] 2-PPN (PPN is Ph3P=N+=PPh3) under nitrogen to afford a mixture of species, among which the key heterobinuclear [Ir–Pt] intermediate [PPN][IrI2(CH3)(CO)2(μ-I)PtI2(CO)] 8-PPN has been identified; [PPN][IrI2(CH3)(CO)2(μ-I)PtI2(CO)] 8-PPN can in its turn lead to the formation of [PPN][PtI3(CO)] 9-PPN, [IrI2(CH3)(CO)2(solv)] 10, [Ir2I2(CH3)2(μ-I)2(CO)4] 3′ and [PPN][Ir2I4(CH3)2(μ-I)(CO)4] 11-PPN; all of these species have been characterized. Under CO pressure, [PPN][IrI2(CH3)(CO)2(μ-I)PtI2(CO)] 8-PPN is a short-lived species that quickly leads to [IrI2(CH3)(CO)3] 4 and [PPN][PtI3(CO)] 9-PPN showing that the main role of the platinum promoter is to abstract an I ligand from [PPN][IrI3(CH3)(CO)2] 2-PPN. Under catalytic conditions, I is abstracted from H[IrI3(CH3)(CO)2] by [PtI2(CO)2] 7 and the rate determining step is accelerated; the relevant species H[IrI3(CH3)(CO)2] 2-H, H[IrI3(COCH3) (CO)2] 6-H and H[PtI3(CO)] 9-H have been observed under 30 bar of CO. A catalytic cycle is proposed, which depicts the cooperative effect between the iridium catalyst and the platinum promoter.