A process for the manufacture of methyl acetate by the carbonylation of dimethyl ether or dimethyl carbonate with carbon monoxide under substantially anhydrous conditions in the presence of a Group TB metal loaded mordenite catalyst prepared by ion-exchanging mordenite with at least one Group IB metal, the ion-exchanged mordenite having a molar ratio of the Group IB metal to total aluminium present in the mordenite in the range 5 to 50 mol %.