Diastereoisomeric mixtures of phosphonodipeptides, prepared from racemic dialkyl 1-aminoalkanephosphonates, were separated by ion-exchange column chromatography. Phosphonodipeptides were found to be much more readily available from dialkyl 1-aminoalkanephosphonates than from the corresponding free acids and the mixed carboxylic – carbonic anhydride method was found to be the most effective one.