A carbonylation process for the production of a carbonylation product such as a carboxylic acid and a carboxylic acid ester by contacting carbon monoxide with a feed comprising an alcohol such as methanol and/or a reactive derivative thereof such as methyl acetate in the vapor phase using a heterogeneous heteropolyacid catalyst which has been ion-exchanged or loaded with at least one metal selected from rhodium, iridium, copper and palladium and a Group IA metal selected from lithium, sodium, potassium and rubidium.