BACKGROUNDDoping nanoceria with zirconia often leads to improved catalytic properties due to enhanced concentration of structural defects in the resulting materials. This study aims to identify the parameters and mechanisms governing the reactivity of well-characterized commercial ceria-zirconia (CexZr1-xO2) nanocatalysts with various Ce contents (x=1, 0.80, 0.50, 0.21, 0) or surface area towards the degradation of Orange II dye in the presence of H2O2. RESULTSDiscoloration and mineralization kinetics were found to be strongly related both to the composition and surface area of the single or mixed oxides. Under standard conditions (23 degrees C, dark, stoichiometric concentration in H2O2), the best discoloration and mineralization performances were achieved for pure ceria (100% of color removal in 2h and 65% mineralization in 8h), but this arises mainly from its higher specific area by comparison with Zr-doped materials. For the first time, the amount of Ce-IV(O-2)(2-) surface species, intermediates in the production of hydroxyl radicals and generated from the reaction of H2O2 with Ce(III) sites, was quantitated using TPD-MS in order to provide a rational basis to explain the catalytic performance. CONCLUSIONDoping ceria with Zr increases the surface density of defective Ce(III) sites that can be converted to Ce-IV(O-2)(2-) species in the presence of H2O2, but other parameters, such as the specific surface area and surface Ce content of the catalysts, are also important. An in-depth mechanism for the reaction of H2O2 on the ceria-zirconia surface is discussed based on the experimental results.