N-mono-substituted derivatives of diamino acids are prepared by the reaction of succinimidyl esters of carboxylic acids or substituted carbonic acids with the unprotected diamino acid. The acylation preferentially occurs at the side chain or terminal amino group of the diamino acid. For example, selective acylation of the terminal amino group of lysine occurs without first having protected the 2-amino group. Such acylation has application in the preparation of inter alia N.sup.6 -palmitoyl-lysine.