This work deals with the search for new molybdate-type catalyst formulations for the activation of light alkanes and their conversion to alkenes. In the first part, we showed that is possible to stabilize the beta-phase of NiMoO4 (as pure phase) by incorporating a certain amount of Co in its lattice. The sol-gel method was also applied to the synthesis of solid solutions of NiMoO4 and CoMoO4. The main difference between the bulk and silica-dispersed Ni-Co-Mo catalysts prepared by citrate or sol-gel methods as well as impregnation, is related to the fact that it is possible to stabilize the beta-Ni1-xCoxMoO4 phase throughout the whole composition range in the dispersed catalysts. Moreover, the catalytic data emphasize the advantage of using mixed Ni-Co molybdates in comparison with simple Ni or Co molybdates and also the fact that a higher activity is reached when these active phases are dispersed in a silica matrix. In the second part, we reported on the synthesis, characterization and catalytic activity of solid solutions of Ni and Co molybdates dispersed on different supports such as Al2O3, MgO, TiO2, ZrO2 and Al2O3-MgO. These experiments demonstrate that supporting or dispersing the mixed Ni1-xCoxMoO4 phase on any kind of support leads to the preferential stabilization of the beta-phase. It was also shown that catalysts characterized by intermediate acid-base properties give better catalytic performances than those with high acidity or high basicity. Bi and/or Ln- (La, Ce, Pr, Sm, Tb) modified Ni, Co and mixed Ni-Co molybdates were prepared, characterized and investigated from the point of view of their catalytic behaviour. The incorporation of a lanthanide in Ni and/or Co molybdate stabilizes the b-phase for citrate-prepared catalysts. Concerning the catalytic activity of these solids, a strong enhancement of the catalytic activity and the selectivity towards propene is observed when solid solutions of beta-NiMoO4 and beta-CoMoO4 are modified by a lanthanide, except in the case of Sm. The Tb-modified catalyst is the one displaying the highest propene productivity, but the most selective catalysts are those modified by La or Pr. Incorporation of Ce strongly improves the catalytic activity, more than propene selectivity. The presence of Bi in the formulation of the catalyst results in a lower propene selectivity.