Amino acids having masked amino groups react with amino acids having masked carboxyl groups to form peptide derivatives in good yields if the reaction medium contains an isonitrile. Isonitriles also are effective in causing condensation of amino acids having masked amino groups with compounds having active hydroxyl groups to the corresponding amino acid esters. The yields of the firstmentioned reaction are enhanced if the reaction mixture additionally contains one of the compounds having active hydroxyl groups. Both the peptide derivatives and the amino acid esters of the compounds having active hydroxyl groups are useful for peptide synthesis. The term ''''amino acid'''' herein embraces also oligopeptides having free amino and carboxyl groups unless specifically stated otherwise.