The transport phenomena in the permeation of acid and salt across anion-exchange membranes used to separate electrolytes by diffusion dialysis (DD) were studied. The experiments and computations were performed as imposed by irreversible thermodynamics. The phenomena are discussed in terms of the phenomenological (L(p), L(D) and L(pD), reflection (sigma) and permeability (omega) coefficients. The negative reflection coefficient indicated facilitated permeation of acid across the anion-exchange membranes. The negative osmotic flow was one of the phenomena increasing the permeation rate of acid. The data suggested that efficient separation by DD resulted from two effects : facilitated transport of one component (an acid) and restrictions in the sorption and permeation of the other one (a salt) by Donnan exclusion.