The catalytic oxidation of propene and toluene has been investigated on pure ZrO2, pure Y2O3 and ZrO2 doped with 1, 5 and 10 mol% Y2O3 in the presence or absence of copper (0.5, 1 and 5 wt.%). A synergetic effect has been detected since ZrO2 and Y2O3 exhibit significant lower activities than the mixed oxides. The higher surface areas, related to structural change from monoclinic (ZrO2) to tetragonal (ZrO2-Y2O3), partly explained the higher activity of ZrO2-Y2O3. However, it has been shown that the number of anionic vacancies, created by the substitution of Zr4+ by Y3+, in yttria-stabilised zirconia solids depends on the yttrium contents. Their effect on propene and toluene oxidation activity is significant. The anionic vacancies should induce a better activity of the ZrO2-5 mol% Y2O3 catalyst with or without copper which presents the higher number of Zr3+ species. This support should favour the formation of CuO particles which should be the most active catalytic sites in the studied reaction. Moreover, whatever the support, copper has a promotional effect on the catalytic activity and the CO2 selectivity. (C) 2002 Elsevier Science B.V. All rights reserved.