The natural mineral montmorillonite is a type of aluminosilicate with a layered structure. Al-O and Mg-O octahedrons are sandwiched between layers of Si-O tetrapedral. The total conductivity of Li-montmorillonite with 15% water content was determined by AC impedance measurement to be 1.04 x 10(-3) S cm(-1) at 25 degrees C, and the activation energy is 0.13 eV. The conductivity of Na-montmorillonite with the same water content was determined to be 2.76 x 10(-3) S cm(-1); the activation energy is 0.11 eV. The results obtained with Ca-, Zn- and Mg-montmorillonites with about 17% water content are 1.27 x 10(-3) S cm(-1), 1.02 x 10(-3) S cm(-1) and 1.21 x 10(-3) S cm(-1), respectively. The activation energies are all about 0.17 eV. By using Wagner’s DC polarization technique, the electronic conductivities of Li- and Mg-montmorillonites were measured to be 4 x 10(-10) S cm(-1) and 3 x 10(-10) S cm(-1). Their ionic conductivities measured here vary with the water content and are associated with outside surfaces. The compensated cations with their hydrated water spheres play the essential role in the conductivity.