Ti- and Nb- containing NiO catalysts have been synthesized by two different preparation methods: i) by precipitation (Me-Ni-O oxides, Me = Nb or Ti), in order to prepare promoted NiO catalysts; and ii) by wet impregnation on TiO2 or NbOx supports, in order to prepare diluted/supported NiO catalysts. The catalysts have been also characterized and tested in the oxidative dehydrogenation of ethane. The catalytic performance of Ti- and Nb-promoted catalysts strongly depends on the composition, although in both cases the optimal one is found at similar Ti or Nb loadings (ca. 90 wt% NiO), showing similar ethylene selectivity in the ODH of ethane (ca. 90% at 10-20% ethane conversion). However, in the case of diluted catalysts, the catalytic behavior of Ti- and Nb-containing catalysts is drastically different. Then, over TiO2-diluted NiO catalyst, the highest selectivity to ethylene (ca. 90% selectivity) is achieved at NiO loading of 20 wt.%. However, over Nb2O5-diluted NiO catalyst, selectivity to ethylene was lower than 70%. A discussion on the characteristics of selective catalysts is done. In this case, the best catalysts must present a low concentration of free NiO and TiO2 or Nb2O5 phases, maximizing the Ni-O-Ti or Ni-O-Nb interaction. Interestingly, this takes place at different NiO loading depending on the preparation method and the nature of promoted/diluter. The low selectivity to ethylene achieved by NiO diluted with Nb2O5 has been related to the low interaction of NiO with the surface of Nb2O5, which hinders the elimination of unselective electrophilic O species.