The interactions between Mo and V on alumina are studied for the oxidative dehydrogenation (ODH) of propane. Dispersed surface molybdena and vanadia species share alumina support but show no interaction below Mo + V monolayer coverage. Vanadia and molybdena species react on alumina into mixed Mo-V-(Al)-O above Mo + V monolayer coverage, which nature depends on environmental conditions. Molybdena sites may form Al-2(MoO4)(3) or Mo-V-O phases depending on loading and temperature. The Mo-V-O phases spread on the support as separate surface oxides at lower coverage, such trend appears promoted by ODH reaction conditions. (C) 2004 Elsevier B.V. All rights reserved.