This paper describes an investigation by EPR spectroscopy and FTIR spectroscopy of the reactivity of ETS-10 photocatalysts subjected to different ion exchange treatments. Trapped photoelectrons can be detected only in ion-exchanged samples which have been damaged as a result of the ion exchange procedure. UV-irradiation of such samples in the presence of adsorbed organic molecules that scavenge photogenerated holes forms Ti3+ ions, which transfer the electron to oxygen to form superoxide O-2- on subsequent addition of O-2(-) Superoxide ions are also formed when damaged samples are irradiated in O-2. As-synthesised (undamaged) samples of ETS-10 are not photo-reduced when irradiated in the presence of adsorbed organic molecules, but do catalyse a novel photo-polymerisation of ethene; the activity for this reaction depends on the level of stacking faults in the ETS-10. Photo-oxidation of ethene to adsorbed carbonyl compounds occurs most extensively in ion exchanged samples; this is attributed to the generation of photocatalytic sites on the external surface of the ETS-10 as a result of the damage caused by ion exchange. (c) 2006 Elsevier B.V. All rights reserved.