Abstract: The impedance plane plots for four ISEs, two liquid membrane bile salt ISEs and two PVC membrane Ca ISEs based on ETH 1001 and phosphate, were measured from 65 kHz to 0.01 Hz. All the impedance spectra obtained showed two semicircles and a Warburg impedance (Fig.1). The bulk resistance of membrane, R_b, and the charge-transfer resistance, R_(ct), were estimated from the high frequency and the low fre-quency semicircles respectively. The activation energy for ion transport in membrane, E_a, and the apparent standard exchange current density, i°_0, were calculaed from the R_b varied with temperature and the R_(ct) varied with the ion concentration in bathing the R_b values are not, and the E_a values and R_b values for two liquid membrane ISEs are also the case, The E_a values for the former are three times larger than the than in liquid membrane. The ETH 1001-based Ca ISE is a very good Nernstain sensor. The i°_0 value of it bathing in Ca~(2+) solution is 1.9×10~(-4) A·cm~(-2). The potentionetric responses of the two liquid membrane ISEs are near Nernstain. The i°_0 of them bathing in the potential-determining ion solution are in 1×10~(-6)-6×10~(-5) A·cm~(-2) range. The Warburg impedance was eliminated by the strong anion surfactant added in bathing solution. It was shown that the diffusion of the potential-determining ion near the membrane/solution interface is inhibited, then the potentialmetric response for the potential-determining ion is disturbed.