Abstract: An improved process for the production of aliphatic or cycloaliphatic dialdehydes or acetals of the same by reaction of an aliphatic or cycloaliphatic conjugated diene with carbon monoxide and hydrogen in the presence of a rhodium catalyst which is modified with a tertiary phosphine or phosphite at a temperature of from 70.degree. to 160.degree. C and at a pressure of from 100 to 600 atmospheres, the improvement consisting in using as catalyst a rhodium complex which contains carbon monoxide, a tertiary organic phosphine or tertiary organic phosphite and a halogen atom as ligands. Aliphatic and cycloaliphatic dialdehydes or the acetals of the same are suitable for the production of the corresponding diols which may be used in the production of polymers and particularly of polyurethanes.