New data on the mechanism of mass transfer in solid-phase reactions were obtained using the electrochemical approach to the synthesis of complex oxides. A manifold decrease in the effective activation energy of the reactive diffusion was observed when the electric field was applied to the MoO3/Pb2MoO5 system. The phenomenon of surface reactive diffusion (SRD) was studied systematically for the first time under model conditions for 14 reactions of molybdate and tungstate syntheses. SRD is a diffusion-controlled process, which is retarded when the front of reactive diffusion reaches a certain critical distance. The effect of electric field on SRD and the overall rate of the interaction are similar. This fact points to the prevailing effect of external potential difference on the surface component of reactive diffusion. The phenomenon of "solid-phase spreading" of molybdenum and tungsten oxides over the surfaces of other oxides in the absence of the chemical reaction was studied. The reasons for the high mobility of MoO3 and WO3 with highly covalent metal-oxygen bonds are discussed.