PEG-NHS ester 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 is necessary to obtain high pegylation yields. The amido succinic acid NHS ester derivative has a C4 amide linkage between PEG and the NHS ester group. It is a comparably reactive PEG active ester with a hydrolysis half-life of a few minutes at pH 8.