The reactive surface diffusion (RSD) kinetics are studied for reactions of a similar nature (the same diffusant, the same major product) but differing in the number of products. The RSD effect is far stronger in the case of a multiphase product; for reactions where strictly one product is formed, the RSD effect is weaker. The promoting influence of microamounts of a minor product on the reaction is interpreted in terms of the microheterogeneous structure of the diffusion zone: when several products are formed upon RSD, new interfaces appear between the major and minor products, the minor product and diffusant, and the minor product and gas. The increased diffusion permeability of these interfaces is the reason for the promotion of the overall RSD flux. A certain role played by minor products in the promotion of the reactive surface propagation is due to the softening of the contact zone as a result of the reduced eutectic melting temperature in a multiphase system. Thermodynamic and kinetic criteria are formulated for reactive surface interdiffusion and the interreversal of the diffusant and substrate functions in surface and intergranular mass transfer. A general model of reactive mass transfer is advanced for processes in which molybdenum, tungsten, and vanadium covalionic oxides are involved. This model, unlike all previous schemes, takes into account additional interfaces and fluxes generated by RSD.