In boreholes, temperatures vary and to extract hydrocarbon saturation from conductivity measurements, the influence of temperature on water and rock conductivities must be accounted for. The mobility (mu DL ) of the counter-ions due to clays and the electrical conductivity of pore-filling brine show large changes with variation in temperature, whereas the microgeometry of the pore space exhibits negligible change. Using this idea, the temperature dependence of mu DL is extracted using data on dc electrical conductivity of shaly sands (sigma ) containing varying amounts of clay. The mobility of Na (super +) counter-ions is found to vary approximately linearly with temperature. This explicit relationship is tested by comparing the predicted temperature dependence against the measured temperature dependence of conductivity of a set of rocks with high and low clay content. While the rock conductivity shows a large temperature dependence, the resistivity index is less sensitive to temperature. An approximate formula, which is superior to Arps's formula, for water conductivity as a function of temperature is obtained.