The long term degradation of polyethersulfone (PES) based ultrafiltration membranes used in the dairy industry has been investigated. The main aim of the study was to identify characterisation techniques which could give an indication of the condition and performance of ultrafiltration membranes after long term exposure to sodium hypochlorite solution. Membranes were degraded using sodium hypochlorite solution at pH 9, 10, 11 and 12, and with 5000 ppm-days to 25,000 ppm-days of exposure at 55°C. The degraded membranes were studied using the following characterisation techniques: dynamic mechanical analysis (DMA), thermogravimetric analysis (TGA), tensile testing, field emission scanning electron microscopy with energy dispersive x-ray spectroscopy (FESEM-EDS), Fourier transform infrared-attenuated total reflectance spectroscopy (FTIR-ATR), contact angle, drop absorption, zeta potential, liquid-liquid displacement porosimetery (LLDP), protein separation, and colour measurement. The protein separation test included membrane throughput measurements (using casein-whey as feed), with size exclusion chromatography and gel electrophoresis to analyse the feed, permeate and retentate. Also a membrane disinfection experiment was performed to study the effect of sodium hypochlorite pH on disinfection of mixed dairy culture.