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An extensive study of the surface chemical changes to poly(ether sulphone) (PES) ultrafiltration membranes is made for the first time by the use of time-of-flight secondary ion mass spectrometry (ToF-SIMS) after photoirradiation at 254 nm with irradiances varying from 10 to 300 mJ cm −2 in a nitrogen atmosphere. Complementary information is provided by analysis with x-ray photoelectron spectroscopy (XPS) and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). The versatility, superior specificity and sensitivity of using ToF-SIMS to investigate degradation phenomena are highlighted. The combined results demonstrate that photoirradiation causes a number of chemical changes to the surface: incorporation of oxygen; degradation of the benzene rings and formation of oxidized carbon species; depletion of carbon; reduction of -SO 2 -to some extent; formation of -OH, C O and -SO 3 H groups; and probable formation of -C 6 H 4 -O-C 6 H 5 end-groups. In addition, no -OSO 3 H groups are formed and no formation of SO 2 is detected. Also, it is shown that chain scission dominates below an irradiation dose of ∼200 mJ cm −2 (at 254 nm in a nitrogen atmosphere). At higher doses, cross-linking becomes dominant.

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