Molybdena/alumina and molybdena/zirconia catalysts with Mo surface densities in the range 1.1-5.2 Mo/nm(2) were studied by in situ vibrational (Raman and FTIR) spectroscopies and O-18/O-16 isotopic exchange experiments combined with in situ Raman spectra at 450 degrees C. The vibrational isotope effects and the combined interpretation of Raman and IR band wavenumbers and characteristics suggest a mono-oxo configuration for the deposited molybdena phase on both catalytic systems at low (similar to 1.5 Mo/nm(2)) as well as at high (4-5 Mo/nm(2)) coverage, irrespective of extent of association (polymerisation). Isolated mono-oxo monomolybdates predominate on alumina at low loadings, while increasing relative amounts of mono-oxo polymolybdates are formed at high loadings. Mono-oxo polymolybdates and monomolybdates occur on zirconia, with the polymolybdates predominating already from low coverage (1.7 Mo/nm(2)). A "next-nearest-neighbor O-18/O-16 substitution" vibrational effect is observed, resulting in small red shifts (2-5 cm(-1)) of the Mo=O-16 Raman band wavenumber. The catalysts' behavior for the oxidative dehydrogenation of ethane is addressed and discussed with relevance to the configuration of deposited molybdena. (C) 2010 Elsevier B.V. All rights reserved.