The invention relates to a modifying agent comprising a water soluble polymer, wherein the water soluble polymer comprises at least one reactive selenium group, said reactive selenium group being capable of reacting with a thiol group thereby forming an —Se—S— bond. Furthermore, the invention relates to a method for producing said modifying agents and their use in the modification of pharmaceutically active agents, e.g. G-CSF. Additionally, the invention concerns conjugates comprising a water-soluble polymer and a pharmaceutically active agent, wherein the water-soluble polymer is linked via a S—Se-bond to agent and a method for their production and their use as medicaments. Finally, the invention concerns a pharmaceutical composition comprising the inventive conjugates.