To improve our understanding of the mechanism of boron extraction from B(OH)(3) aqueous solutions by means of electrodialysis across a homogeneous anionic ADP/216 membrane, the conductivity of the berate form of the membrane was investigated using impedance spectroscopy. Measurements were carried out for concentrations ranging from 0.02 to 0.51 mol kg(-1) at four temperatures between 5 and 40 degrees C. The marked decrease in the membrane conductivity at higher concentrations was attributed to the formation of univalent polyborate anions, the mobilities of which decrease with increasing number of boron atoms. Equilibrium constants for the formation of tri- and pentaborate species were estimated. For this membrane, the specific conductivities at 25 degrees C of the mono- and triborate counter-ions were found to be (30 +/- 1) Ohm(-1) cm(-1) and (0.30 +/- 0.05) Ohm(-1) cm(-1) respectively. The activation energies for migration of these two species were proportional to the number of boron atoms.