High-pressure NMR mechanistic investigations into the promoting role of dicarbonyldiiodoplatinum complex in the iridium-catalysed carbonylation of methanol to acetic acid are consistent with iodide abstraction from the [IrI3(CH3)(CO)2]– resting state species by [PtI2(CO)2], unbolting the rate-limiting migratory CO insertion step. A new [Ir–Pt] heterodimetallic complex, which presumably represents the key species involved in the abstraction of I–, has been characterised by NMR in tandem with FAB-MS.