Current efficiency for electric transfer of boric acid across two types of anionic exchange membranes are reported and related to the condensation of boric acid in solution. Equilibrium properties of boron species in the exchanger have been derived from the variation of the membrane capacity for boron exchange and the sorption of boric acid measured in the concentration range 0-0.55 mol dm-3 of bathing solution. Boron diffusion through the membrane, either in chloride or in borate forms, allows a comparison of passive and facilitated diffusion processes in both types of membranes. Equilibrium and diffusion parameters thus obtained permit the interpretation of electrodialysis experiments which could demonstrate that, for a given concentration of the donor solution, the current efficiency is governed strictly by the average number of boron atoms per exchanging site of the membrane.