A process for the in-situ regeneration of a zeolite catalyst in a carbonylation process for the production of at least one of methyl acetate and acetic acid by contacting methanol, dimethyl ethyl or dimethyl carbonate with carbon monoxide in the presence of a zeolite catalyst, ceasing contact of the catalyst with the reactant, regenerating the catalyst with a regenerating gas selected from hydrogen or a mixture of hydrogen and carbon monoxide at a temperature in the range 250 to 600° C., terminating the hydrogen regenerating step and resuming contact of the catalyst with the reactant and carbon monoxide.