The effect of treatment with different mineral acids (H2SO4, H3PO4, HNO3 and HQ on the activity of monolithic CoOx/gamma-Al2O3 catalysts in the reduction of nitric oxide with methane in the presence of oxygen (CH4-SCR of NOx) was studied. Their behaviour in the methane oxidation reaction in both the presence and absence of NOx was determined in order to interpret the results in terms of intrinsic activity and competition between both processes. Depending on the nature of the acid used, significant differences were observed in the catalytic activities which were related to the textural states, surface acidities and the nature of the detected species. The best results were obtained after treatment with H2SO4, which increased the activity towards NOx elimination compared to the other catalysts. This behaviour was attributed not only to an increase in surface acidity but also to the stabilisation of the active Co2+ species, thus avoiding the formation of Co3O4 spinel that is responsible for the strongly adsorbed NOx species that lead to NO2 formation which increase the rate of the undesired methane oxidation reaction at high temperatures. (c) 2006 Elsevier B.V. All rights reserved.