An empirical adsorption isotherm proposed in 1909 by German–American physical chemist Herbert Max Finlay Freundlich (1880–1941) that relates the quantity of gas molecules adsorbed onto a surface at a constant temperature of the form:

where θ is the measure of the sites occupied per unit area of surface measured as the ratio of the mass of adsorbate to the mass of adsorbent, p is the equilibrium pressure of the adsorbate, and k and n are empirical constants for a particular temperature. When 1/n approaches zero, the adsorption becomes independent of pressure. The isotherm is often applied to the adsorption from liquid solutions and is useful when considering the performance of heterogeneous catalysis.