It is proposed that interfacial reaction barriers in binaryA/Bdiffusion couples lead to the absence of phases predicted by the equilibrium phase diagram, provided that the diffusion zones are sufficiently thin (thin‐filmcase). With increasing thickness of the diffusion zones the influence of interfacial reaction barriers decreases and the simultaneous existence of diffusion‐controlled growth of all equilibrium phases is expected (bulkcase). Selective growth of the first and second phases and the effect of impurities are discussed with the influence of interfacial reaction barriers and with references to the known cases of silicide formation.