A series of Cu/mordenite catalysts were prepared by competitive ion exchange, characterized by temperature programmed reduction with hydrogen, temperature programmed desorption of NO, adsorption of pyridine and tested for the selective catalytic reduction of NO by decane in presence of a large excess of oxygen. At low Cu content (exchange% less than or equal to 53%), the TPR profiles show the presence of two peaks of comparable areas, the accessibility to Cu sites is high (NO/Cu > 0.8) and the micropore volume remains unchanged, thus suggesting that Cu2+ ions remain isolated and are reduced in two successive steps : Cu2+ --> Cu+ at about 510 K and Cu+ --> Cu-o at 600-700 K. At higher Cu content (exchange% > 60%), the TPR profiles are modified with a large increase of the low temperature peak; the accessibility to Cu sites decreases (NO/Cu = 0.45-0.49), and the micropore volume also decreases, thus suggesting the formation of Cu oxide dusters in the lattice of the mordenite. The acidity of the mordenite also decreases upon exchange. The activity for the NO reduction is proportional to the number of Cu2+ ions up to 53% exchange, then decreases. Isolated Cu2+ ions appear then more active than CuO clusters or protons for the selective reduction of NO by decane.