The formation of biofilms in a waste paper medium was studied in a pilot plant by analysing the redox potential in the biofilm. Miniaturised redox electrodes were applied at the reactor wall/biofilm phase boundary. With this measurement set-up, it was possible to demonstrate the effectiveness of biocides and thus to avoid under- and over-doses with these agents. The redox signals measured were correlated with reference methods, such as colony-forming units and dehydrogenase activity.