During intermixing in phase separating A/B diffusion couple the actual atomic fractions of A atoms (c alpha, and c beta) on the left as well as on the right hand side of the alpha/beta interface gradually will decrease (from unity) as well as increase (from 0) until the equilibrium c alpha, and c beta will be reached (alpha and beta refer to the A- and B -rich phases, separated by the interface). At the same time the interface will also be shifted. Assuming that the atomic flux across the interface, J., is proportional to the deviations from the equilibrium (Delta(alpha) = C alpha - C beta and Delta beta = C beta - C beta) the J1= (1/Omega)[K-I beta alpha Delta(alpha)+K-I beta alpha Delta(beta)] relation is obtained, where Omega= Omega A = Omega B is the atomic volume. It is shown that the KI alpha beta and KI beta alpha interface transfer coefficients are positive, independent of the interface velocity, equal to each other for symmetric miscibility gap, and can be given as KI alpha beta = KI alpha beta = K = Z(v)a Gamma C-I alpha beta(alpha)xi.Here Gamma(I alpha beta) is the jump frequency from alpha to beta phase across the interface, a is the lattice spacing, z(v) is the vertical coordination number xi= [1+exp (ZV(C alpha - C-beta)/kT)], where Z is the coordination number, V is the well-known solid solution parameter, proportional to the heat of mixing, and kT has its usual meaning. The above expression justifies the conjecture, frequently used in the literature, that only one interface transfer coefficient is enough for the description of the mass transfer across an interface. For short diffusion times the finite value of Gamma(I alpha beta) will restrict the flux, leading to finite permeability with linear kinetics of interface shift. It is also shown that for an order of magnitude estimation of the critical interface shift (giving the transition from interface to diffusion control) the x(c) = a Gamma(C-beta)/Gamma(I alpha beta), relation can be used, where F(C-beta) is the jump frequency in the beta hase at C-beta. Thus x(c) is practically independent of the value of V and only the composition dependence of the jump frequencies is important. For composition independent jump frequencies x(c) congruent to a, (i.e. it cannot be detected), while for the case when the diffusivity changes by seven orders of magnitude from pure A to B, x(c) is about 150 nm. (C) 2017 Elsevier Ltd. All rights reserved.