This International Standard specifies the determination of the overall1) specific external and internal surface area of disperse (e.g. nano-powders) and/or porous solids by measuring the amount of physically adsorbed gas according to the method of Brunauer, Emmett and Teller (BET method) , based on the and 2015 International Union for Pure and Applied Chemistry (IUPAC) recommendations. Gas adsorption allows for assessing surface area of nonporous materials and porous materials with accessible porosity (which depends on the chosen adsorptive). The BET method is applicable only to adsorption isotherms of type II (disperse, nonporous or macroporous solids) and type IV (mesoporous solids, there are however) (see Figure 1. Type II and Type IVa isotherms. However, in the case of Type IVb isotherms, caution is required since pore condensation may occur at quite low p/p0 (see 2015 IUPAC recommendations, ref. . The BET method cannot reliably be applied to solids which absorb the measuring gas. A strategy for BET area determination of microporous materials (type I isotherms) is described in Annex C.