A series of 10%Mo/Si:Ti = 1:1 catalysts modified with chlorine (Cl/Mo = 0.1-2.0) were studied with regard to their activity for the oxidative dehydrogenation of ethane. The effect of chlorine on the redox and adsorption characteristics and, in turn, on the catalytic performance has been investigated. From X-ray diffraction, temperature-programmed reduction and electron spin resonance experiments, it was seen that the addition of chlorine could optimize the redox characteristics of surface molybdena species, i.e. depress the reduction potential of Mo5+, resulting in increased selectivity for ethylene. Temperature-programmed desorption following ethane adsorption showed an increased ease of ethylene desorption from the surface. TPRxn DRIFTS experiments after ethane adsorption, which pointed to a stronger interaction of ethane with the catalyst and increased surface olefinic species suggest that chlorine cannot only affect the further reaction of ethylene over MoOx catalysts, but also change the interaction of the catalyst with ethane, altering ethane activation steps. (C) 2004 Elsevier B.V. All rights reserved.