Ni-W-O mixed oxides were prepared through the evaporation of aqueous solutions of nickel nitrate and ammonium tungstate and calcined in air at 500 degrees C for 2 h. The catalysts were characterized by several techniques (N(2) adsorption, X-ray diffraction, temperature-programmed reduction, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy of adsorbed CO, and (18)O/(16)O isotope exchange) and tested in the oxidative dehydrogenation of ethane. The catalytic activity and catalyst reducibility decrease when the W content increases. Thus, nickel sites seem to be the active centers for ethane activation in these catalysts. However, the selectivity to ethylene strongly changes depending on the Ni/W ratio. In W-rich catalysts, in which NiWO(4) and WO(3) are mainly observed, a strong influence of ethane conversion on the selectivity to ethylene is observed. However, in Ni-rich catalysts, in which NiO crystallites and WO(x) nanoparticles are mainly observed, ethane conversion hardly influences the selectivity to ethylene. It has been demonstrated that the nature of the Ni sites and the characteristics and number of the acid sites determine the catalytic behavior of these catalysts. The presence of Lewis acid sites with high acid strength in W-rich catalysts facilitates the decomposition of ethylene during ethane oxidation. (C) 2011 Elsevier Inc. All rights reserved.