This International Standard specifies the determination of the overall (see Note) specific external and internal surface area of disperse (e.g. nano-powders) or porous solids by measuring the amount of physically adsorbed gas according to the Brunauer, Emmett and Teller (BET) method (see Reference [1]). It takes account of the International Union for Pure and Applied Chemistry (IUPAC) recommendations of 1984 and 1994 (see References [7][8]).