PEG crosslinking agent 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. This PEG linker with a maleimide endgroup is applied in sulfhydryl-selective coupling of PEG to proteins and other thiol substrates and for selective scavenging of thiol containing peptides. The maleimide group reacts at about neutral pH with free thiols via a Michael addition to form a stable thioether bond.