The present invention relates to a process for the separation of a mixture of a protein and its reaction product with a polyalkylene glycol, comprising the steps: a) providing a mixture comprising the protein and the reaction product in a fluid, b) bringing the mixture of step a) into contact with a microporous cellulose acetate membrane with adsorption of the reaction product onto the cellulose acetate membrane, the protein not becoming adsorbed onto the cellulose acetate membrane, c) removal of the protein from the cellulose acetate membrane, and d) desorbing the reaction product from the cellulose acetate membrane, the reaction product being selected from the group comprising the protein which is mono- or polyfunctionalized by the polyalkylene glycol, or mixtures thereof.
