Methoxy PEG propionaldehyde is a stable aldehyde PEG that can be used for protein, antibody, peptides or carbohydrate N-terminal amine group pegylation reagent. The reaction between aldehyde group with the α-amine at the N-terminus produces an intermediate Schiff base. Further reduction with hydride will form a stable C-N bond. PEG aldehydes react with n-terminus amine groups at a pH of from 5.0 to 9.5. Higher pH will result in multiple Pegylation with both terminal and lysine groups.