It is proposed that the structure of a new layerAB, which forms between the componentsAandB, is defined by parameters of two‐stage interfacial reactions. The two stages, namely transition ofAatoms through theAB/Bboundary and rearrangement of atoms inBintoAB, have independent rates. In the initial stage of the interaction between the two components the flow through the thin new layer does not control the reaction rate. The high rate of atomic flow through the interface combined with the relatively low rate of diffusion at the interface will result in the formation of metastable (amorphous, quasicrystalline, or crystalline) phases. The additional growth of the layer results in a decrease in the growth rate down to a critical value. The thickness of the metastable layer is thus limited and a stable crystalline phase starts to form at the interface. The critical thickness of the metastable phase depends on the diffusion parameters of the initial and intermediate phases.