This method is for determining the surface area of porous substances based upon the Brunauer-Emmett-Teller1 (B.E.T.) theory of multilayer adsorption through the use of measurements at two partial pressures using nitrogen as the adsorption gas. Pore volume is determined according to the capillary condensation theory.2 Pore diameter is calculated from surface area and pore volume. The method is limited to substances with surface areas greater than 10 m2/g and pore diameters less than 60 nm. This method uses barometric pressure as the reference pressure while UOP Method 821 is referenced to nitrogen saturation pressure. Therefore, surface areas determined by this method are typically as much as 10% lower than those produced by UOP 821.