Thiol-PEG-Hydrazide (HS-PEG-HZ) is a linear heterobifunctional PEGylation reagent with a thiol and a hydrazide. It is a useful crosslinking or bioconjugation reagent with a PEG spacer. Thiol, or SH, sulfhydryl, or mercapto group selectively reacts with maleimide, OPSS, vinylsulfone and transition metal surface including gold, silver, etc. Hydrazide can react with carbonyl compounds including ketone or aldehyde to form an acyl hydrazone linkage, which is pH sensitive and can be used for reversible PEGylation. Hydrazine PEG is also used to PEGylate carboxyl, carboxylic acid groups.