A process for the carbonylation of dimethyl ether, dimethyl carbonate and/or methanol with carbon monoxide and hydrogen in the presence of a mordenite catalyst to produce at least one of acetic acid and methyl acetate in which process the mordenite catalyst is regenerated in-situ by contacting the catalyst with a regenerating gas comprising a molecular oxygen-containing gas and an inert diluent at a total pressure in the range 1 to 100 bar and wherein the partial pressure of the molecular oxygen-containing gas is such that the temperature of the catalyst is maintained within the range 225 to 325° C.