The determination of the specific surface area and pore structure parameters of natural materials have been a long-standing issue. We propose a method for determining the specific surface area and pore size distribution that is based on the zeta adsorption isotherm and apply that method for each of two hydrocarbon vapors, octane and heptane, adsorbing on two types of shale, as-received and milled shale. We determined the specific surface areas of both materials, approximate the pores as cylindrical, and determine the average pore mouth radius. The standard deviation in the mean values of the specific surface areas and average pore radius determined with each of the two vapors is less than 3%. The experimental isotherms on the as-received shale indicate the existence of a pronounced adsorption-desorption hysteresis loop that results from the mesoporous structure of the material. We previously showed that liquid forms in the pores because of coalescence of molecular clusters inside the pores. In this study, we show that the pore emptying in the as-received shale is delayed because of the pore blocking effect. The zeta isotherm theory and the necessary conditions for thermodynamic equilibrium along with the measured amount adsorbed are used to determine the distribution in the pore mouth radii.