MMoO4/SiO2 composites (M=Ni or Co) were prepared by a sol-gel procedure involving Si(OMe)(4), M(NO3)(2) and (NH4)(6)Mo7O24 as Starting materials. The dried gels were treated at increasing temperatures until crystalline grains of MMoO4 highly dispersed in the amorphous silica matrix were formed (675 degrees C). They were evidenced by XRD and TEM measurements. Beside supporting the MMoO4 mixed oxide, the silica xerogel appears to play the important role of stabilizing the beta-phase of nickel molybdate, which otherwise would turn into the a-phase at room temperature. The catalytic activity of the beta-NiMoO4/SiO2 composite in the oxidative dehydrogenation of isobutane was compared with related systems containing the metastable beta-molybdate.