Acid-PEG3-NHS ester is a PEG derivative containing a carboxylic acid and an NHS ester. The hydrophilic PEG spacer increases solubility in aqueous media. The terminal carboxylic acid can be reacted with primary amine groups in the presence of activators (e.g. EDC, or DCC) to form a stable amide bond. The NHS ester can be used to label the primary amines (-NH2) of proteins, amine-modified oligonucleotides, and other amine-containing molecules.