Azido-SS-PEG2-acid is a PEG derivative which includes an azide, a carboxylic acid and a cleavable disulfide bond. The azide group can undergo copper-catalyzed Click Chemistry reactions with alkynes, DBCO and BCN moieties. 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 PEG spacer increases the water solubility of compounds in aqueous media.