Preparation of an aliphatic carboxylic acid having (n+1) carbon atoms, where n is an integer up to 6, and/or an ester or anhydride thereof may be achieved by contacting an aliphatic alcohol having n carbon atoms and/or a reactive derivative thereof with carbon monoxide substantially in the absence of the halogens or derivatives thereof at a temperature in the range 250-600° C. and at a pressure in the range 10 to 200 bars, in the presence of a catalyst consisting essentially of a mordenite which has, as framework elements, silicon, aluminum and one or more of gallium, boron and iron, and which has been ion-exchanged or otherwise loaded with copper, nickel, iridium, rhodium or cobalt.