Adipic acid is selectivity prepared by reacting water and carbon monoxide with at least one pentenic acid, in the presence of a catalytically effective amount of an iridium-based catalyst and at least one iodinated promoter therefor at an elevated temperature, at a pressure greater than atmospheric and in at least one solvent medium which comprises a saturated aliphatic or cycloaliphatic hydrocarbon or halogenated derivative thereof, an aromatic hydrocarbon or halogenated derivative thereof, or an aliphatic, aromatic or mixed ether, and wherein the I/Ir atomic ratio is less than 20.