Photoactivated processes occurring upon UV illumination of CeO2 nanoparticles prepared from reverse microemulsions have been studied by means of EPR spectroscopy. Different experimental approaches have been employed aiming to determine the capability of the samples for the trapping, of photogenerated charge carriers either in the absence or in the presence of external scavengers or spin-trapping agents (O-2 or DMPO molecules). These results are discussed in connection with the photocatalytic activity obtained over the samples for toluene oxidation. Under the employed experimental conditions, hydroxyl and superoxide radicals are detected as main species resulting from hole and electron trapping, respectively, at the surface of the samples. The photocatalytic activity of the samples can be related to the efficiency in the charge separation processes, in accordance with EPR results of oxygen photoadsorption. However, the nature of the active species is uncertain since no correlation is found between the activities for generation of hydroxyl radicals, based on DMPO trapping EPR experiments, and the photooxidation rates. This suggests the direct involvement of holes or other type of hole-derived entities as active species. (C) 2004 Elsevier B.V. All rights reserved.