Eifert et al have recently reported that the aluminum contents in the α⁄β interface in Cu-Al diffusion couples are higher than those given by the equilibrium phase diagram. Similar experiments on the α⁄γ interface in γ Fe-Cr diffusion couples in this work have proved that the chromium contents in the interface are considerably lower than those in α and γ phases of the equilibrated two-phase alloys. These experiences suggest that the condition of interphase equilibrium is affected by the rate of atom transfer across the phase interface. A phenomenological consideration has been given to the rate of atom transfer concerning the chemical potential gradient and the atomic mobility in the interface layer. The mobility of the α⁄γ interface in the γ Fe-Cr system has been estimated to be about 1/500 of that of lattice diffusion in the α phase. This concept has also been applied to analyze a result on the non-equilibrium α⁄α interface found in α Fe-Cr diffusion couples heated at a temperature between 830° to 910°C for a long time, and the reason why the equilibrium phase γ is missing has been discussed.