The photodegradation of poly(ether sulphone) (PES) was investigated systematically by time-of-flight SIMS (ToF-SIMS) and x-ray photoelectron spectroscopy (XPS). The effect of varying the irradiation dose, wavelength and the atmosphere was studied along with mechanistic photooxidation studies using O-18(2). Photoinduced -SO3H group formation was observed at 199, 260 and 315 nm for an oxygen-containing atmosphere only. Photoinduced -OH group formation is observed at all investigated wavelengths and atmospheres. Photoinduced cross-linking was an important process in a vacuum. The oxygen reacts quickly with the PES surface during photoirradiation and chain scission was found to be the dominant process, and oxygen incorporation was found to be non-specific but pronounced for -SO3H group formation. It was found that for conditions where processes are affected by photoirradiation the extent is observed to follow the UV absorption and showed little dependence on the wavelength. The wavelength only affects the extent of the process in question, which is ascribed to a fast equilibrium of the absorbed energy. The photoinduced processes are very sensitive towards the presence of the smallest amounts of oxygen in the atmosphere. Copyright (C) 2004 John Wiley Sons, Ltd.