Process for producing acetic acid by carbonylating methanol and/or a reactive derivative thereof in a liquid reaction composition in which there exists in equilibrium, at least a first soluble catalytic species and a second soluble catalytic species. The first catalytic species is the least catalytically active or promotionally active of the species existing in the equilibrium. The process includes determining (i) the concentration of the first catalytic species and/or (ii) the ratio of the concentration of the first catalytic species to the concentration of the second catalytic species in equilibrium therewith, present in the liquid reaction composition and/or a present in a liquid fraction in a separation step, and maintaining (i) and/or (ii) below a pre-determined value.