A process for the co-production of acetic anhydride and acetic acid which process comprises introducing a carbonylatable feedstock comprising methyl acetate and/or dimethyl ether and optionally also comprising methanol and/or water, to a carbonylation reactor in which there is maintained a liquid reaction composition comprising acetic anhydride, acetic acid, rhodium carbonylation catalyst, alkyl iodide co-catalyst and an iodide salt promoter consisting essentially of an alkali metal iodide and/or alkaline earth metal iodide, contacting said carbonylatable feedstock with carbon monoxide in said liquid reaction composition to produce acetic anhydride and acetic acid, and introducing to the carbonylation reactor methyl formate and/or formic acid in the range from 0.1 to 20% by weight of the total feed of liquid components to the reactor.