PEG-Thiol derivatives are useful for surface modification or reversible pegylation via a reducable S-S bond. Thiols exhibit a high affinity towards metal ions and metal surfaces like silver, gold etc. Mercapto PEGs are widely used for pegylation of metal surfaces, films or nanoparticles. The free thiol group acts as a soft nucleophile and reacts readily with maleimide functions to form stable thioether bonds. On the other hand it reacts with other free thiols to form cleavable S-S disulfide bonds.