A simple, relatively precise apparatus which permits absorption measurements to be made at pressures from about 10 torr to about 40 atm is described. It involves the use of a transducer or a pressure cell. The use of a transducer, because of its small internal volume and the accuracy with which its output can be read potentiometrically, gives this apparatus an advantage over one in which pressures are measured by a conventional Bourdon-tube gauge. When surface areas of about 50 m² are to be measured, about 120 cm³ s.t.p. of a gas such as xenon or carbon dioxide are required and the surface areas (calculated from the equation of Branauer, Emmett and Teller) are capable of duplication to within ±2%.