Polyethersulfone/polyvinylpyrrolidone (PES/PVP) ultrafiltration membranes are widely used to produce safe drinking water. Chemicals such as bleach are necessary to clean membranes; however, they are suspected of playing an important role in membrane ageing. To improve our understanding of how hypochlorite cleaning solutions affect filtration membranes, PES, PVP and thin films of PES/PVP blends have been aged by soaking in bleach water. This study was performed using 4000 ppm of free chlorine with a controlled pH of 8 and 12. Any modifications of the chemical structure of the thin films (w40 mm) were analysed using infrared spectroscopy and any modifications to the molecular weight of the polymers were determined via size-exclusion chromatography. In addition, the involvement of a radical oxidation process was investigated by contacting the materials with OH radicals generated by the irradiation of a titanium dioxide suspension. The results demonstrated that the degradation of PES/PVP blends in bleach water (sodium hypochlorite solution) involved radical oxidation of the PVP component, which promoted oxidation of the PES component. This work allowed a mechanism for the degradation of hollow fibre ultrafiltration membranes based on PES (95%) and PVP (5%) in contact with hypochlorite cleaning solutions to be proposed.