The permeability values of "TN" oganomineral nanofiltration membranes to water, L, and to supercritical CO2, G, were compared. The resulting values for G were an order of magnitude higher than for L. The difference may be directly related to the viscosity difference between the two fluids. Temperature- and pressure-related variations in G were also analyzed; for this purpose, Poiseuille’s model satisfactorily accounts for experimental behavior, while Knudsen’s model is unsuitable. A hysteresis effect was observed on the isotherms corresponding to variations in G versus pressure, suggesting partially irreversible CO2 adsorption on the micropore walls, that would diminish the radius. This phenomenon could be enhanced by an increased fluid density and viscosity.