The membrane potential and conductance of a liquid membrane, with dicetylphosphoric acid as a mobile carrier, were measured as functions of the pH of the outside solution, containing HCl and KCl, while maintaining constant electrolyte concentrations of the inside solution. The effects of membrane solvents with different carbon numbers (l-hexanol, l-octanol, and l-decanol) were investigated. The transference numbers of each ion were estimated and the overall conductance was resolved into ionic conductances of H + and K +. Approximate expressions for the membrane potential and conductance were derived by solving the Nernst-Planck equations, taking into account the Donnan potentials at the membrane/solution interface. For a highly charged liquid membrane, the membrane potential was found to be almost equal to the difference between the Donnan potentials at the two membrane/water interfaces; the contribution from the diffusion potential across the membrane was negligibly small. By curve-fitting of the experimental data on the membrane potential and conductance, the mobility of the carrier, U s , in the membrane was estimated. Also the equilibrium constants were obtained for the association reaction between K + and the carrier, K K, and for the association reaction between H + and the carrier, K H. The mobility of the carrier U S was shown to be close to the mobility of K +. The association constants, K K and K H, increased by a factor of about 10 as the solvent carbon number increased by two. The selectivity of the membrane for H + over K +, expressed by K H b H K K b K (where b is the partition coefficient of ionic species between the membrane and aqueous phases), increased with the increase in the carbon number.