t-Boc-aminooxy-PEG3-propargyl is a click reagent with a propargyl group and t-Boc-aminooxy group. The propargyl group can react with azide-bearing compounds or biomolecules via copper catalyzed Click Chemistry. T-Boc-aminooxy group can be free up under mild acidic conditions. The hydrophilic PEG spacer increases solubility in aqueous media.