Bromoacetamido-PEG4-acid is a PEG derivative containing a bromide group and a terminal carboxylic acid. The hydrophilic PEG spacer increases solubility in aqueous media. The bromide (Br) is a very good leaving group for nucleophilic substitution reactions. 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.