t-Boc-N-amido-PEG4-NHS ester is a PEG derivative containing a Boc-protected amine group and an NHS ester group. The hydrophilic PEG spacer increases solubility in aqueous media. The NHS ester can be used to label the primary amines (-NH2) of proteins, amine-modified oligonucleotides, and other amine-containing molecules. The protected amine can be deprotected by acidic conditions.