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We comment on a recent paper in this journal by El Fallah et al.1 It is generally acknowledged that ceria plays an important role in three way catalysts (TCW).2 In particular, the investigation of the ceria oxidation state under different thermal and chemical conditions constitutes a major challenge in the characterization of the TCWs and related catalytic systems.3-6 A number of techniques have been applied with this purpose: EPR,7-8 XPS,9-11 electronic conductivity measurements,12 oxygen pulses/TPO,13'14 magnetic balance,15-21 UV-vis spectroscopy,20 and even FTIR spectroscopy.22 Very recently, X-ray absorption spectroscopy (XAS) has also been used.1 As is shown in ref 1, XAS may constitute a very interesting alternative technique. Both quantitative and kinetic aspects of the problem can fruitfully be investigated.1 In addition, the studies can be performed under various thermal and chemical environments, which is also critically important. 16 If ref 1, the redox behavior of some bare ceria samples as well as of a Rh/Ce02 catalyst, under both reducing and oxidizing conditions, has been investigated. To interpret their results, the authors1 proposed the following reaction scheme.H, k,

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