Mal-PEG4-acid is a PEG derivative containing a maleimide group with a terminal carboxylic acid. The hydrophilic PEG spacer increases solubility in aqueous media. The maleimide group will react with a thiol group to form a covalent bond, enabling the connection of biomolecule with a thiol. 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.