Voltammetry of immobilized microparticles (VMP), diffuse reflectance UV-vis spectroscopy (DRS), X-ray powder diffraction (XRD), and Raman spectroscopy were used to identify V(V) oxidic species in vanadia catalysts supported on titania, alumina, mesoporous silicas, and composites of Al(2)O(3) and TiO(2) with montmorillonite (MMT). V(2)O(5) was detected by VMP irrespective of its crystallinity at concentrations close to the detection limit of crystalline V(2)O(5) by XRD. The interpretation of DRS was re-evaluated after VMP analyses, because detection of lower concentrations Of V(2)O(5) by DRS is complicated by partial overlap of their absorption bands with rather intense absorption bands of major oligomeric vanadate species. while sensitivity Of VMP to V(2)O(5) is much better than to less condensed vanadates. VMP selectively identified poly-vanadates prevailing on TiO(2) supports and oligomeric vanadate species prevailing oil alumina at medium V-loadings. XRD-nanocrystalline V(2)O(5) was found as major species on impregnated mesoporous silicas with V loading above 4 wt.%. Al- and Ti-chemically modified MMTs disperse V(V) oxide species preferentially either on the Al(2)O(3) or TiO(2) components, or on the aluminosilicate components; these cases are easily distinguished by VMP. VMP, especially if combined with spectral and XRD analyses. can produce detailed information about V(V) oxidic forms at V-loading close to or slightly above the critical monomer monolayer concentration, where 2-D and 3-D condensation of [VO(4)] units is initiated. These forms play an important role in functionality of heterogeneous catalysts in oxidative dehydrogenation of ethane. (C) 2009 Elsevier B.V. All rights reserved.