This paper contains the results of detailed research study to determine the factors which govern the electrical conductivities of oil sands and rocks. The experimental work shows the electrical resistivity to be an inverse function of the percentage of conductive water present in any rock type. It was found that the resistivity-moisture curve of both petroliferous and non-petroliferous rock are of a general hyperbolic form. For the higher values of moisture content, the curves rapidly approached the conductivity of the electrolyte contained within the rock or oil sands, while for the lower values of moisture content the resistivity values are high and governed by the properties of the rock. Rocks containing soluble salts have critical points on their resistivity-moisture curves, with the greatest change occurring in the neighborhood of ten per cent moisture content. Rocks containing fresh water have relatively high resistance values which change fairly uniformly with the variations in moisture content. The research work indicates that the presence of oil does not appreciably effect electrical resistance of the rock. Since the resistivity values are dependent upon the electrolytic effect, the work indicates that resistivity values alone are not a reliable criterion for predicting the presence or absence of an oil sand. The work explains further why high electrical resistivity values are not to be expected when applying electrical geophysical methods for mapping subsurface structure.