Process for the preparation of 3-phenylpropanal in which cinnamaldehyde is hydrogenated with the aid of a Pd-containing catalyst in the presence of a small amount of water. A high yield and a high selectivity are obtained in a commercially attractive process. The reaction mixture obtained can without intermediate further processing be subjected to an oxidation in which the cinnamaldehyde is oxidized to 3-phenylpropionic acid with a high degree of conversion and a high selectivity. The combination of the two process steps consitutes a simple, commercially attractive process for the preparation of 3-phenylpropionic acid using cinnamaldehyde as a starting material. 3-phenylpropionic acid can be used in particular in the preparation of pharmaceuticals such as HIV protease inhibitors.
