We present a new procedure to determine the geometric area, accessible pore volume, and pore size distribution of activated carbon, and we test this with a detailed computer simulation study of a number of porous solid models. For these model adsorbents with known atom configurations, we determine the "intrinsic" accessible volume, the surface area, and pore size distribution using the Monte Carlo integration method proposed by Herrera et al. (Herrera, L.; Do, D. D.; Nicholson, D. A Monte Carlo integration method to determine accessible volume, accessible surface area and its fractal dimension. J. Colloid Interface Sci. 2010, 348 (2), 529-536). The inverse problem postulates that the theoretical adsorption isotherm is a linear combination of local isotherms, and matches the theoretical isotherm to the "computer-experimental" adsorption isotherm. The results suggest that this method is a promising tool to determine structural parameters of a porous solid. As a corollary, we propose a definition for the absolute isotherm as an alternative to the excess isotherm used in the literature.