The reduction of nitrogen monoxide by propene on V2O5/ZrO2 doped with or without calcium has been studied by FTIR spectroscopy as well as by analysis of the reaction products. Considerable promoting effect of calcium doping on the reduction of nitrogen monoxide by propene was observed on the V2O5/ZrO2 catalysts. For the reaction of a mixture of NO + C3H6, carbonyl and carboxylate species were observed above 373K, although nitrate species formed at room temperature on V2O5/ZrO2 doped with calcium. No bands due to a compound including both carbon and nitrogen atoms were observed. Thus, the redox mechanism, i.e, propene reduces the catalyst and nitrogen monoxide oxidizes the catalyst, is confirmed on V2O5/ZrO2 catalysts doped with or without calcium. The analysis of the V=O band in the region of 1100-900 cm(-1) indicates that this promotion is mainly due to new V=O species formed by the addition of calcium onto the catalyst. This species is easily reproduced in comparison with the other V=O species on the surface in the reoxidation process of the catalyst. (C) 2001 Elsevier Science B.V. All rights reserved.