Technical Abstract: The electrical conductivity in soils is often related to soil solution salinity, but contributions from charged colloids and exchangeable cations may be also important. The purpose of this study is to examine the influence of smectite properties on frequency-dependent complex electrical conductivity, EC, of hydrated clays. We used a vector network analyzer to determine the EC under alternating electrical fields for frequencies ranging from 300 kHz to 3 GHz. Four smectite clays were saturated with Ca, Mg, Na, or K and equilibrated at four relative humidities (RH) ranging from 56 to 99%. X-ray diffraction analysis was used to determine the amount of 1, 2, 3, or 4 layers of water in the interlayers. Each EC spectra was fit to an equation that included the d.c. EC, the frequency corresponding to the change in slope, and the slope of the log-log plot at the higher frequency end. Saturating cation, smectite properties, and RH significantly affected fitted components of the spectra. Low water content increased tortuosity of the flow path and reduced EC. As EC decreased, the high frequency slope increased. For each cation -- clay combination, EC was always correlated with increases in internal water content. Highly charged smectite clays and associated cations can have a large EC which should not be ignored in soil and sediment studies.