Mal-amido-PEG2-acid is a PEG linker containing a maleimide group and a terminal carboxylic acid. The hydrophilic PEG spacer increases solubility in aqueous media. The terminal carboxylic acid can react with primary amine groups in the presence of activators (e.g. EDC, or HATU) to form a stable amide bond. The maleimide group will react with a thiol group to form a covalent bond, enabling the connection of biomolecule with a thiol.