Alkali metal salts of tetraphenylboron dissociate in aqueous solution to yield the hydrophobic anion, BPh-4, which is strongly adsorbed at the surfaces of lipid bilayer membranes. Upon application of a transmembrane voltage pulse these anions cross the membrane without appreciable desorption, thereby exhibiting a transient electrical conductance. The relaxation time of this transient is governed by the height of the central potential barrier which the anions must surmount in crossing the membrane. Because of discrete charge effects, the barrier height and hence the observed relaxation time increase markedly with increasing surface density of adsorbed BPh-4. Since adsorbed BPh-4 are in partition equilibrium with the same species dissolved in the aqueous phase, measurement of the relaxation time for BPh-4 membrane conductance can be used to assay the aqueous-phase concentration of the hydrophobic anion. In this way we have observed the precipitation of KBPh4 in water, obtaining a solubility concentration product of 1.0 X 10(-7) mol2 X dm-6 for the precipitation reaction at 25 degrees C. This result is larger by a factor of two than the most directly comparable published values from other sources. In additional experiments we have reduced the polarity of the aqueous phases bathing the membrane by adding varying amount of ethylene glycol to the water. Using the same conductance relaxation assay, we have determined that partitioning of BPh-4 into the membrane/solution interfaces is lessened as the polarity of the bathing solutions is reduced. This result is attributed to a lowering of the chemical potential of the BPh-4 in the less polar medium.