t-Boc-Aminooxy-PEG5-amine is a PEG derivative containing a Boc-protected aminooxy group and an amino group. The hydrophilic PEG spacer increases solubility in aqueous media. The protected aminooxy can be deprotected under mild acidic conditions and then can be reacted with an aldehyde or ketone group to form a stable oxime linkage. The amino group is reactive with carboxylic acids, activated NHS esters, carbonyls (ketone, aldehyde) etc.