The application of semiconductor photocatalysis in waste water treatment has been intensively investigated over the past decade. Most studies involve titanium-based photocatalysts; however, practical applications are still limited by their poor visible light activity. As an alternative, a tungsten trioxide-based photoelectrocatalytic fuel cell (PECFC) with a cell configuration based on the proton exchange membrane fuel cell (PEMFC) technology has been employed for pollutant remediation. In this study, the degradation of a persistent chlorophenol (2,4-dichlorophenol) was assessed using a visible light active tungsten trioxide photocatalyst. The degradative progress of the 2,4-DCP was monitored over a period of 24h by both chemical analysis and a bacterial biosensor (Escherichia coil HB101 pUCD607) toxicity assay. A 74% decrease in concentration of the 2,4-DCP was observed after a period of 24 h, from which ca. 54% were accountable to degradation processes and 20% due to pollutant losses by adsorption or volatilisation. The biosensor toxicity response correlated well with the observed concentration reduction of 2,4-DCP, but also indicated the formation of more toxic intermediates. HPLC-MS analysis was carried out to study intermediate degradation products. There were indications of the occurrence of stable dimers within the first few hours of the degradation process and the formation of intermediate degradation products of higher bacterial toxicity than the parent compound. Although the potential of the PECFC, as a sustainable method for water treatment, has been demonstrated, further work is required to optimise this new technology for mineralisation of organic contaminants. (C) 2012 Elsevier B.V. All rights reserved.