Ald-Ph-PEG3-O-NH-Boc is a PEG derivative containing an aldehyde group and a Boc protected aminooxy group. The protected aminooxy can be deprotected under mild acidic conditions and can be further reacted with an aldehyde or ketone group to form a stable oxime linkage. 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.