Ald-CH2-PEG5-azide is a crosslinker containing an azide group and an aldehyde group. The azide group enables PEGylation via Click Chemistry. The hydrophilic PEG spacer increases solubility in aqueous media. The aldehyde group reacts with amines and the N-terminal of peptides and proteins to form an imine containing a C=N double bond, which can be further reduced. PEG aldehyde or ketone derivatives can be used in reversible PEGylation through its reaction with hydrazine or hydrazide forming a hydrolytic acyl hydrozone linkage.