PEG di(carboxylic acid) or (Bis-PEG-acid) is a class of homobifunctional PEG linkers which will react with an amine-containing moiety in the presence of a coupling reagent. PEG di(carboxylic acid) (Bis-PEG-acid) can be used to modify proteins, peptides and other materials via amino or other acid reactive chemical groups via PEGylation. It can also be converted to N-hydroxysuccinimide active esters which can be widely used in bioconjugation.