Methylamino-PEG2-acid HCl salt is a PEG derivative containing a carboxylic acid with a methylamine group. The hydrophilic PEG spacer increases solubility in aqueous media. The methylamine group is reactive with carboxylic acids, carbonyls (ketone, aldehyde) etc. 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.