Active-Mono-Sulfone-PEG8-acid is a PEG derivative with a sulfone and acid end group. The sulfone group can be conjugated with thiol groups of proteins. The terminal carboxylic acid can be reacted with primary amines in the presence of activators such as EDC and DCC to from stable amide bonds. The hydrophilic PEG chain increases the water solubility of the compound in aqueous media.