The structural and catalytic properties of MoO3 catalysts supported on ZrO2, Al2O3, TiO2 and SiO2 with Mo surface densities, n(s), in the range of 0.5-18.5 Mo/nm(2) were studied for the oxidative dehydrogenation (ODH) of ethane by in situ Raman spectroscopy and catalytic activity measurements at temperatures of 400-540 degrees C. The molecular structure of the dispersed surface species evolves from isolated monomolybdates MoO4 and MoO5, depending on the support) at low loadings to associated MoOx, units in polymolybdate chains at high loadings and ultimately to bulk crystalline phases for loadings exceeding the monolayer coverage of the supports used. The nature of the oxide support material and of the Mo-O-suppon bond has a significant influence on the catalytic behaviour of the molybdena catalysts with monolayer coverage. The dependence of reactivity on the support follows the order ZrO2 > Al2O3 > TiO2 > SiO2. The oxygen site involved in the anchoring Mo-O-support is of relevance for the catalytic activity. (C) 2007 Elsevier B.V. All rights reserved.