In a process for the continuous production of acetic acid by feeding methanol and/or a reactive derivative thereof and carbon monoxide to a carbonylation reactor wherein there is maintained a liquid reaction composition comprising methyl acetate, water, Group VIII noble metal carbonylation catalyst, hydrocarbyl halide co-catalyst, optionally at least one promoter, and acetic acid, the methyl acetate concentration in the liquid reaction composition is maintained at a pre-determined value by monitoring the ratio of methanol and/or reactive derivative thereof to carbon monoxide being converted to acetic acid and adjusting the methanol and/or reactive derivative thereof feed rate in response thereto in a manner such that the methyl acetate concentration is maintained at the pre-determined value. This can assist in reducing build up of methyl acetate in the reactor during periods of loss of activity.