Methanol is converted to acetic acid by reaction with carbon monoxide in the presence of an carbonylation system which comprises a rhodium catalyst component and a liquid reaction medium containing acetic acid, methyl iodide, methyl acetate, and at least one pentavalent Group VA oxide and water in specific concentrations. The present carbonylation system not only increases the yields and reaction rates but also serves to stabilize the rhodium catalyst component in an active form.