TCO-PEG3-Maleimide enables simple and efficient incorporation of TCO moiety onto thiol-containing molecules, including antibodies and cysteine-containing peptides. The maleimide group specifically and efficiently reacts with reduced thiols (sulfhydryl groups, –SH) at pH 6.5- 7.5 to form stable thioether bonds. The polyethylene glycol (PEG) spacer arm is hydrophilic and improves solubility in aqueous buffers.