Methyltetrazine-PEG4-Maleimide is a PEG derivative containing a maleimide group and a hydrophilic PEG spacer which increases water-solubility. The maleimide group specifically and efficiently reacts with thiols to form stable thioether bonds. This reagent can be used to efficieintly incorporate a tetrazine moiety onto antibodies, cysteine-containing peptides and other thiol-containing molecules.