A process for the production of acetic acid by carbonylating with carbon monoxide methanol and/or a reactive derivative thereof in a carbonylation reactor containing a liquid reaction composition comprising an iridium carbonylation catalyst, methyl iodide co-catalyst, a finite concentration of water, acetic acid, methyl acetate and at least one promoter wherein the water concentration is at or below that at which the maximum in the graph of carbonylation rate versus water concentration occurs and there is employed in the liquid reaction composition a co-promoter selected from alkali metal iodides, alkaline earth metal iodides, metal complexes capable of generating I.sup.-, salts capable of generating I.sup.-, and mixtures of two or more thereof.