Synergetic effects were observed in the oxidative dehydrogenation of ethane to ethylene, when Mo-V-O or Ni-V-O catalysts are mechanically mixed with alpha-Sb2O4. Synergy was observed on the conversion, the yield and the selectivity in ethylene. XRD and XPS analyses showed that a chemical reaction between these oxides occurred during the reaction. The role of this contamination was evaluated taking into account results presented in the literature (namely, formation of Mo-Sb-O and Ni2Sb2O6) and experimentally, by preparing V-Sb-O and mixing it with Ni-V-O. It is concluded that the formation of a Mo-Sb-O phase is excluded and that contamination induced by V-Sb-O alone cannot explain the synergetic effect observed. The more plausible explanation of the synergy should be the existence of a cooperation between Mo-V-O or Ni-V-O with alpha-Sb2O4. It is proposed that the role of added alpha-Sb2O4 (and Ni2Sb2O6, if formed) would be to reoxidize the surface of Mo-V-O and Ni-V-O during the reaction, thanks to a spillover of oxygen.