In order to evaluate the effect of sulfate ions in zirconia-supported cobalt catalysts on the reactivity of adsorbed NOx species toward methane, experiments involving the interaction of the hydrocarbon with NOx-free and NOx-precovered catalysts are performed. The nitrate species formed at room-temperature adsorption of NO/O-2 over the CoOx/ZrO2 catalysts are inert toward the methane in the 573-723 K temperature range. Over NOx-precovered CoOx/SO42--ZrO2 catalysts, oxidation of the hydrocarbon starts at 573-623 K with the participation of reactive nitro-nitrato species coordinated to cobalt sites. It is proposed that in the catalytic reduction of NO over the sulfated cobalt catalysts, the activation of methane occurs on cobalt sites and the products of the latter process-formate species and formic acid-are key intermediates capable of selectively reducing the nitro-nitrato species. (C) 2004 Elsevier Inc. All rights reserved.