The effect of Cs content in Cs-Cu/ZrO2 and Cs-Co/ZrO2 systems was studied in NOx reduction by toluene. From physicochemical characterization data, it was shown that the formation of CuO crystallites is favoured following the Cs content in Cs-Cu/ZrO2 catalysts whereas cesium tends to hinder the formation of Co3O4 crystallites in Cs-Co/ZrO2. Regarding their catalytic properties, Cs-Cu/ZrO2 catalysts with low Cs content are more active than Cs-Co/ZrO2 ones in NOx reduction by toluene. The difference is partly due to the formation of high NO2 amount over cobalt based catalysts. (c) 2012 Elsevier B.V. All rights reserved.