Aminooxy-PEG4-CH2CO2tBu is a PEG derivative containing an aminooxy group and a t-butyl protected carboxyl group. 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 t-butyl protected carboxyl group can be deprotected under acidic conditions.