Methyltetrazine-amido-PEG5-alkyne is a PEG derivative containing an alkyne group and a hydrophilic PEG spacer which increases water-solubility. This reagent enables simple and efficient incorporation of a tetrazine moiety onto antibodies, cysteine-containing peptides and other thiol-containing molecules. The alkynes can be reacted with azide-bearing compounds or biomolecules via copper catalyzed azide-alkyne Click Chemistry to yield stable triazole linkages.