t-Boc-Aminooxy-PEG4-alcohol is a PEGreagent with a Boc-protected aminooxy group and an alcohol group. The hydrophilic PEG spacer increases solubility in aqueous media. The protected aminooxy can be deprotected under mild acidic conditions and then can react with an aldehyde to form a stable oxime linkage. The hydroxyl group enables further derivatization or replacement with other reactive functional groups.