PEG bis-aldehydes are used for crosslinking amino substrates. They react with free amines, peptides and proteins forming a hydrolytically unstable imine bond, which can be reduced afterwards or insitu by hydrides to form a stable secondary amine bond. With hydrazide substrates, hydrolyzable hydrazone bonds are formed. Therefore reversible pegylation is possible. Reaction with alkoxyamines forms stable oxime bonds.