Insoluble monolayers of nonionic or ionic surfactants at the air/water interface are assumed to be made up of singly dispersed molecules and clusters of different sizes. A surface equation of state is derived combining the Gibbs adsorption equation with an expression for the chemical potential derived on the basis of the above model. The equation can be used to describe the changes with nonhorizontal lines in the pi-A isotherms. The equation is predictive, since expressions are proposed for all the free-energy contributions to the free energy of formation of the clusters. The surface equation of state is used to interpret experimental data regarding the behavior of pentadecanoic, hexadecanoic, and octadecanoic fatty acids as well as the behavior of sodium octadecyl sulfate.