beta + gamma mixed Bi-Mo-O phases with a large range of composition were prepared by coprecipitation for investigating the existence of synergetic effects between beta and gamma phases for the oxidative dehydrogenation of 1-butene.Results from surface characterization show that the surface of the catalysts always has a composition similar to the beta phase but the gamma phase grows around beta phase. Mixed phases present higher activity and less CO2 selectivity than pure beta and gamma phases. This result leads to propose that the whole catalyst (bulk and surface) participates in the catalytic process.The synergetic effect between these two phases can be ascribed to the high oxygen mobility in the lattice of the gamma phase that allow the migration of oxygen species from the gamma to the beta phase. (C) 2003 Elsevier B.V. All rights reserved.