Adipic acid is prepared by a process which comprises the following steps:(a) Hydroformylation of a pentenoic ester by reaction with carbon monoxide and hydrogen at elevated temperatures and under superatmospheric pressure in the presence of a cobalt carbonyl or rhodium carbonyl complex with formation of a mixture of 5-, 4- and 3-formylvaleric esters,(b) isolation of the 5-formylvaleric ester from the resulting mixture of 5-, 4- and 3-formylvaleric esters, a mixture essentially consisting of 4- and 3-formylvaleric esters remaining,(c) dehydrocarbonylation of the mixture consisting essentially of 4- and 3-formylvaleric esters in the presence of one or more elements of subgroup 8 of the Periodic Table at from 50.degree. to 400.degree. C. with formation of pentenoic esters, and recycling of the latter to stage (a) for hydroformylation,(d) oxidation of the 5-formylvaleric ester from stage (b) with molecular oxygen or a gas containing molecular oxygen to give a monoester of adipic acid, and(e) hydrolysis of the monoester of adipic acid to give adipic acid.