IN the determination of surface area using nitrogen at the temperature of liquid nitrogen by the method of Brunauer, Emmett and Teller1, it is usual to determine the dead-space on each sample, using helium. For routine determinations on a substance known to give S-shaped isotherms, that is, where the equation of Brunauer, Emmett and Teller is applicable, it is possible to avoid this dead-space determination. If V1 c.c. nitrogen are adsorbed at a pressure p1, then the dead-space is given by where D is dead-space at unit pressure and a is a correction factor equal to 6.58 × 10- 5 at unit pressure. Hence, if Vc1 is volume of nitrogen in the sample bulb, then It follows that