The surface area determined by BET is thus normally larger than the surface area determined by air permeability. The method used complies with Ph. Eu. Types II, IV and VI can be measured by BET method. (interaction adsorptiv- adsorbent > adsorptiv-adsorbate). Types III, V have weak interactions between gas. by the BET method. G. FAGERLUND (1). The theoretical background of the adsorption isotherm equation of Brunauer, Emmett and Teller is reviewed, together.