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. PEG-NHS derivatives are active esters and useful reagents to pegylate proteines, peptides or any other surfaces with a free primary amino group to form stable amide bonds. The Reaction requires a neutral to alkaline pH (7 to 8.5) to proceed. However, hydrolysis competes with aminolysis of the active ester and also increases with higher pH. Thus, working with excess PEG-NHS ester is necessary to obtain high pegylation yields. The PEG butyric acid NHS ester derivative has a C4 chain directly attached to PEG. It is one of the most stable PEG NHS esters known with a hydrolysis half-life of over 20 min at pH 8.