A range of aryl, alkyl and heterocyclic aldehydes have been treated with hydrogen cyanide in the presence of the ‘Inoue’ catalyst, (R, R)- or (S, S)-cyclo[phenylalanylhistidyl]. Most aryl aldehydes with electron-donating substituents in the m- or p-positions give high enantiomeric excess (e.e.) values (=80%) but aryl aldehydes with strong electron-withdrawing substituents gave moderate e.e. values (=50%). These moderate values are believed to be due to partial racemization of the product cyanohydrins in the presence of the mildly basic catalyst. In contrast to the reactions of aryl aldehydes, reactions of alkyl aldehydes and of ketones gave low e.e. values (=30%) and an explanation is proposed.