The resolution of racemic 1-(4-methoxybenzyl)-octahydro-isoquinoline, a key intermediate in the synthesis of the antitussive agent dextromethorphan, is reported using (R)-2-(6-methoxy-2-naphthyl) propionic acid in good yields. The resolving agent and the undesired isomer of the octahydro-isoquinoline have been recovered in good yield.