Two diffusion routes and locations of the reaction front are observed during the interaction of MoO3 or WO3 with MexOy in MoO3 / MexOy, cells. The first front is located at the direct contact between the reagents (thickness l). The second front propagates on the external side surface of MexOy. (length l(s), thickness delta). The latter is due to the surface reaction diffusion (SRD). More than 30 reaction systems were studied (Me = Ca, Cd, Pb. Ni, Zn, Cu, Co, Ln). The nature and specific features of SRD caused by a low surface energy, a high surface mobility, specific forms of migration of MoO3 and WO3, and a double layer present at the initial interface were discussed.