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. Carboxy PEGs may be used for conjugation to amine substrates (forming amides) or other nucleophiles (forming esters) . However, activation is necessary prior to conjugation. Different PEG carboxy derivatives display a different degree of stability and reactivity of the corresponding active ester. 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.