Electron transfer between two transition metals, MoCl5 and VOx supported on silica, is studied in situ at the gas-solid interface by spectroscopy (electron paramagnetic resonance, diffuse reflectance UV-vis-nearIR, XPS, Raman), electron microscopy, and chemical analysis for a long period of time (similar to9000 h). Initially, the VOx/SiO2 (V-V, 3d(0)) solid phase is composed of V2O5 crystallites and monomeric tetrahedral vanadyl species grafted onto silica ((dropSiO)(3)V=O). Upon reaction at 30 degreesC with gas-phase MoCl5 (Mo-V, 4d(1)), the solid undergoes drastic changes in color, which define four stages. Each stage is accompanied by variations in the chemical state of the supported phase. The principal results are as follows. (i) No electron transfer and no reaction takes place between MoCl5 and grafted (dropSiO)(3)V=O species. (ii) In contrast, V2O5 crystallites undergo both electron transfer from Mo-V to V-V and chemical changes leading to a change in the speciation of V and to the formation of a new and amorphous V-IV-O-Mo-VI-Cl phase. (iii) The following sequence of redox potentials can be proposed for the MoCl5-VOx/SiO2 system: E(V-V/V-IV)(V2O5) (cryst) > E(Mo-VI/Mo-V)(MoCl5) > E(V-V/V-IV)((dropSiO)3V=0). The difference in behavior of the two types of vanadium species toward electron transfer from MoCl5 is discussed in terms of the dependence of the nature of the ligands, the symmetry, and the nuclearity of the vanadium species on the redox potentials.