Vanadia transport, which is a minor reaction flux in the solid state reaction between V2O5 and MoO3, was studied using chemical and neutron activation analyses and electron spectroscopy for chemical analysis. It was found that negligible quantities of vanadia were transferred in a molybdena briquette during the reaction. Vanadia was presumably localized in thin external layers of molybdena grains. The reaction potential difference U-r across a Pt \ MoO3\V2O5 \Pt Cell was studied. It was shown that in this cell U-r was produced at the molybdena briquette and was due to vanadia transport. The U-r value changed with time in two stages. The reaction potential difference U-r was constant (or diminished slightly) at the first stage and dropped abruptly at the second stage. The duration of the first stage depended on the initial thickness of the MoO3 briquette: the thicker the briquette, the longer the U-r value was nearly constant. Causes and probable mechanisms of U-r generation are discussed in different terms: chemical reaction, variation of a(O2) at the boundary between the reaction product and initialoxides, or surface spreading of the minor (V2O5 or V9Mo6O40) diffusant. The last mechanism, which received the least study in the general case, was shown to be the most probable one for the reaction at hand.