Abstract: An apparatus applying the dynamic BET-type approach to the measurement of solid surface areas, allowing the pressure on the sample to be varied from a lower, subatmospheric value to a plurality of upper values either greater than or less than atmospheric pressure. Apparatus comprises, in the order of their appearance from upstream to downstream, the following parts: a source of a mixture of gaseous adsorbate and non-adsorbable gas, a pressure reducing valve, a reference circuit and a measurement circuit arranged in parallel, said measurement circuit incorporating a sample holding a cell and both said circuits going through a katharometer, then converging into a vacuum pump. A flow control valve is installed upstream from the sample holding cell and a throttling valve is inserted between said sample cell and the katharometer.