t-Boc-Aminooxy-PEG2-CH2CO2H is a PEG derivative containing a Boc-protected aminooxy group and a terminal carboxylic acid. 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. The hydrophilic PEG spacer increases solubility in aqueous media. The Boc-protected amine can be deprotected under mild acidic conditions.