Aminooxy-PEG3-acid HCl salt is a PEG derivative containing an aminooxy group and a terminal carboxylic acid. The hydrophilic PEG spacer increases solubility in aqueous media. The aminooxy group can be used in bioconjugation. It reacts with an aldehyde to form an oxime bond. If a reductant is used, it will form a hydroxylamine linkage. The terminal carboxylic acid can be reacted with primary amine groups in the presence of activators (e.g. EDC, or DCC) to form a stable amide bond.