Disclosed is a process for the production of acetic acid or mixtures of acetic acid and acetic anhydride in a carbonylation process wherein a mixture comprising methyl acetate and/or dimethyl ether and methyl iodide is contacting in the liquid phase with carbon monoxide in the presence of a carbonylation catalyst at elevated pressures and temperatures. Methanol, water, or a mixture thereof is added to an acetic anhydride-containing stream within a flash evaporation zone to convert some or all of the acetic anhydride to acetic acid and optionally methyl acetate and to provide heat for the evaporation of a portion of the product effluent produced by the carbonylation process.