Molybdena/zirconia catalysts with surface densities, n(5), in the range of 1.7-10.5 Mo/nm(2) were studied by in situ Raman spectroscopy and simultaneous measurement of their catalytic performance for the oxidative dehydrogenation of ethane at temperatures of 420-540 degrees C. Isolated monomolybdates [O=(Mo-O)(4)] and polymeric MoOx units with distorted octahedral (CNMo = 5) configuration coexisted on the support at various proportions depending on loading; bulk Zr(MoO4)(2) crystals were formed at surface densities exceeding the monolayer (>7 Mo/nm(2)). The surface composition and the structural properties of the dispersed species were found to respond to changes in the catalyst gas environment; by exploiting the Raman band intensities, it was concluded that reduction of Mo=O and Mo-O-Mo sites under steady state-reaction or reducing conditions was facilitated with increasing loading and that the Mo=O sites were perturbed and reduced to a greater extent compared with Mo-O-Mo. The selective reactivity to ethylene was found to increase up to the monolayer coverage, with high ethylene selectivity persistent at high conversions. The combination of structural and catalytic data revealed a concurrence between the trends for the apparent activity per Mo atom and for the number of Mo-O-Zr bonds per Mo versus n5. The anchoring Mo-O-support bonds were shown to be of relevance for the catalytic activity. (C) 2008 Elsevier Inc. All rights reserved.