The iridium/iodide-catalyzed carbonylation of methanol to acetic acid is promoted by carbonyl complexes of W, Re, Ru, and Os and simple iodides of Zn, Cd, Hg, Ga, and In. Iodide salts (LiI and Bu4NI) are catalyst poisons. In situ IR spectroscopy shows that the catalyst resting state (at H2O levels ≥ 5% w/w) is fac, cis-[Ir(CO)2I 3Me]-, 2. The stoichiometric carbonylation of 2 into [Ir(CO)2I3(COMe)]-, 6, is accelerated by substoichiometric amounts of neutral promoter species (e.g., [Ru(CO) 3I2]2, [Ru(CO)2I2] n, InI3, GaI3, and ZnI2). The rate increase is approximately proportional to promoter concentration for promoter: Ir ratios of 0-0.2. By contrast anionic Ru complexes (e.g., [Ru(CO) 3I3]-, [Ru(CO)3I3] -, [Ru(CO)2I4]2-) do not promote carbonylation of 2 and Bu4NI is an inhibitor. Mechanistic studies indicate that the promoters accelerate carbonylation of 2 by abstracting an iodide ligand from the Ir center, allowing coordination of CO to give [Ir(CO)3I2Me], 4, identified by high-pressure IR and NMR spectroscopy. Migratory CO insertion is ca. 700 times faster for 4 than for 2 (85 °C, PhCl), representing a lowering of ΔG‡ by 20 kJ mol -1. Ab initio calculations support a more facile methyl migration in 4, the principal factor being decreased π-back-donation to the carbonyl ligands compared to 2. The fac, cis isomer of [Ir(CO)2I 3(COMe)]-, 6a (as its Ph4As+ salt), was characterized by X-ray crystallography. A catalytic mechanism is proposed in which the promoter [M(CO)mIn] (M = Ru, In; m = 3, 0; n = 2, 3) binds I- to form [M(CO)mIn+1] -H3O+ and catalyzes the reaction HI (aq) + MeOAc → Mel + HOAc. This moderates the concentration of HI(aq) and so facilitates catalytic turnover via neutral 4.