The study of the defective surface sites of many oxides has received considerable interest, as these sites are coordinately unsaturated and exhibit extraordinary activity in many catalytic reactions. 1 Most of the research work in this particular area is mainly focused on theoretical simulations. 2 However, the influence of the defect sites on the chemical nature of the oxide surface and their role in the mechanism of activation of a substrate have not yet been fully explored. In this Communication, we take the example of ceria and provide direct evidence wherein the defect-site-enriched oxide promotes the surface reorganization and thereby influences the activation of ethylbenzene (EB) in the oxidative dehydrogenation in presence of N 2 O.Ceria is an interesting oxide and is one of the constituents in many catalyst formulations. 3 The method of preparation strongly influences the structural and surface properties of ceria. 4 We intend to explore its catalytic activity in the oxidative dehydrogenation (ODH) of ethylbenzene using N 2 O. A nanocrystalline ceria sample enriched with Ce 3+ -O --Ce 4+ -type defect sites was prepared by the alcoholysis method (ceria-A). For comparison, three other ceria samples were prepared, one by conventional precipitation and the other two by a solution-combustion method using urea and glycine as fuels (ceria-P,-U and -G), respectively, as reported elsewhere. 4 Interestingly, the nanocrystalline ceria prepared by the alcoholysis method exhibits Ce 3+ -O --Ce 4+ -type defect sites predominantly confined to the surface, as evidenced from diffuse reflectance UVvisible spectroscopy (a weak band at around 650 nm). 5 A rough estimate of the surface to bulk defect ratio follows the order Ceria-A > Ceria-G > Ceria-U/P based on the EPR spectroscopic analysis 6 (Supporting Information, Figure S1) and the temperature programmed reduction 7 of the samples ( Figure S2). These defect sites promote the dehydrogenation of EB using N 2 O at 598 K, which is far lower than the temperatures normally encountered in the existing conventional processes for the dehydrogenation of EB (873 K). An equilibrium conversion of 45 mol % and a styrene selectivity of 94%, comparable to that obtained in commercial practice are achieved by the ceria catalyst prepared by the alcoholysis method. However, the ceria samples prepared by conventional and combustion methods showed comparable activity and selectivity to styrene at much higher activation temperatures (Ceria-G at 648 K and Ceria-U/P at 723 K).To address the role of defect sites in governing the temperature of activation, EPR investigations were done on the ceria samples by subjecting each one of them to hydrogen treatment, as the dehydrogenation process involves interaction of hydrogen atoms with the catalyst surface. Figure 1 shows the direct correlation between the concentrations of Ce 3+ -O --Ce 4+ -type defect sites [at g ⊥ ) 1.96 and g | ) 1.933 (D signal) and 1.940 (A signal)] and the temperature of activation. In the case of defect-site-enriched ceria-A...