An isotherm that takes account of the possibility that the monolayer in the Langmuir adsorption isotherm can act as a substrate for further adsorption. The BET isotherm (named after Stephen Brunauer, Hugh Emmett, and Edward Teller who derived it in 1938) has the form:
where z = p/p* (p* is the vapour pressure above a macroscopically thick layer of liquid on the surface), Vmon is the volume that corresponds to the surface being covered by a monolayer, V and p are the volume and pressure of the gas respectively, and c is a constant. In the BET isotherm, the isotherm rises indefinitely at high pressures (in contrast to the Langmuir isotherm). It provides a useful approximation over some ranges of pressure but underestimates adsorption for low pressures and overestimates adsorption for high pressures.