PEG crosslinking agents for biotinylation. The biotinyl functionality is used to attach avidin or streptavidin containing substrates. Pegylation by binding of biotinyl PEG to avidin and streptavidin is very specific and fast. The butyric acid endgroup is directly connected to the PEG chain via a very stable ether linkage. After activation it is used to conjugate the PEG derivative to nucleophilic substrates, mainly amines (to form amides) or hydroxyls/thiols (forming esters) . PEG butyric acid active esters are one of the most hydrolytically stable PEG active esters. They can be used for pegylation in aqueous media without the need of a large excess of PEG.