t-Boc-Aminooxy-PEG12-t-butyl ester is a PEG derivative containing a Boc-protected aminooxy group and a terminal carboxylic acid. The hydrophilic PEG spacer increases water solubility in aqueous media. The Boc-protected amine can be deprotected under mild acidic conditions. The t-butyl protected carboxyl group can be deprotected under acidic conditions.