t-Boc-N-amido-PEG5-NHS ester is a PEG derivative containing an NHS ester and a Boc-protected amino group. The hydrophilic PEG spacer increases solubility in aqueous media. The Boc group can be deprotected under mild acidic conditions to form the free amine. The NHS ester can be used to label the primary amines (-NH2) of proteins, amine-modified oligonucleotides, and other amine-containing molecules.