Mixed Ni-Co molybdate catalysts dispersed on silica and modified by Bi or lanthanides (La, Ce, Pr, Sm and Tb) are prepared according to a sol-gel procedure and tested for their activity in propane oxydehydrogenation. Attempts to rationalize the global increase of catalytic activity towards partial and total oxidation are presented, that are based on correlations with fundamental physico-chemical properties of the additional elements: ionization energy, absolute hardness and redox potential. The incorporation of lanthanides in the formulation of these catalysts is shown to allow a fine tuning of the local electronic properties while keeping the same overall oxide lattice structure, due to the similarity of their chemical behaviour and ionic size. (C) 2004 Elsevier B.V. All rights reserved.