Allysine is a very important crosslink precursor in connective tissue proteins. We describe the reactions of Phthalyl-Allysine-p-Nitrobenzylester, which is a suitable compound to investigate the reactions of the aldehyde group in vitro. Crosslinked compounds are synthesized by mixing suitable stoichiometric ammounts of Phthalyl-Allysine-p-Nitrobenzylester and nucleophiles in aqueous organic solvents. The results are compared with the reaction pathways which have been proposed for crosslink synthesis in vivo.