a process for the optical resolution of - 2 - hydroxy - 4 - phenylbutanoic acid which comprises treating - 2 - hydroxy - 4 - phenylbutanoic acid with an optically active 1 - ethylamine or an optically active n - ethyl - α - methylbenzylamine as a resolving agent is provided by the present invention . according to the invention , - 2 - hydroxy - 4 - phenylbutanoic acid , which is useful as a starting substance for the synthesis of angiotensin converting enzyme - inhibiting pharmaceuticals , can especially be obtained in a highly pure state and in a high yield .