In this paper the authors examine for the first time the relationship between the time of establishment of the equilibrium potential, characterized by the response time tau approximately equals t//6//3 or its multiples 3 tau equals t//9//5, 5 tau approximately equals t//9//9 (the times in which the electrode potentials reach 63, 95, and 99% respectively of the equilibrium potential), and by the conductivity of LaF//3 single-crystal membranes, which is characterized by the concentration of vacancies in the anionic sublattice. The fluoride-ion concentration, in turn, is directly linked with the concentration of the impurity differing in valence. It was shown experimentally for ion-selective electrodes based on LaF//3 single crystals with different CaF//2 contents that the response time decreases with increase of the ionic conductivity of the membranes. It was found that the calculated parameter tau //1, characterizing electrochemical transport of ions through the membrane-solution interface, depends significantly on the ionic conductivity of the membranes.