Substituted phenylpropanals and a process for their manufacture by reaction of the corresponding phenylethenes with carbon monoxide and hydrogen at temperatures of from 50.degree. to 180.degree. C and pressures of from 20 to 1,500 atmospheres gauge, in the presence of rhodium carbonyl complexes. The phenylpropanals according to the invention have pleasant scents. 3-(4-Isopropyl-phenyl)-butanal-1 and 3-(4-tert.-butyl-phenyl)-butanal-1, in particular, have suitable scents for the preparation of flower oil. They are also valuable starting materials for synthesizing drugs, plant protection agents and dyes.