A product comprising a C1-C3 aliphatic carboxylic acid or corresponding ester is produced by a process comprising carbonylating a C1-C3 aliphatic alcohol or a corresponding ester with carbon monoxide in the presence of a.zeolite catalyst having an 8-member ring channel which is interconnected with a channel defined by a ring with greater than or equal to 8 members, the 8-member ring having a window size of at least 2.5 Angstroms×at least 3.6 Angstroms and at least one Brønsted acid site and the zeolite having a silica:X2O3 ratio of at least 5, wherein X is selected from aluminium, boron, iron, gallium and mixtures thereof with the proviso that the zeolite is not mordenite or ferrierite.