A considerable influence of relative humidity on the conductivity of sodium and lithium-montmorillonite was found. At high humidity the conductivity values are similar for both type of samples and higher than those measured at low humidity, whereas the activation energies show the opposite relationship. In the low humidity zone the lithium-containing samples exhibit higher conductivity than that of the sodium ones. An explanation is given based on the different nature of the sorbed water and on the hydration power of the Li+ and Na+ ions.