Carboxymethyl-PEG-Carboxymethyl, MW 5K is a PEG derivative which contains two carboxylic acid groups. The terminal carboxylic acids can be reacted with primary amine groups in the presence of activators (e.g. EDC, or DCC) to form a stable amide bond. The hydrophilic PEG spacer increases the water solubility in aqueous media.