.delta.-Formylvalerates are prepared by a process wherein a 3-pentenoate is treated with an isomerization catalyst at elevated temperatures and a 4-pentenoate is distilled off, and the resulting 4-pentenoate is reacted with carbon monoxide and hydrogen at from 80 to 160.degree. C. under from 1 to 30 bar in the presence of a rhodium carbonyl complex which has been modified with tertiary organic phosphines or phosphites.