Phospholipid monolayers at an oil/water interface are treated as two-dimensional regular solutions made up of three components, namely, singly dispersed phospholipid molecules, clusters of phospholipid molecules, and "empty" sites occupied by water and oil molecules. A simple surface equation of state is derived and used to explain the phase transition that occurs in the monolayer. The equation contains two terms : one is due to the two-dimensional mixing entropy and the other one arises from intermolecular interactions calculated in the nearest-neighbor approximation. The equation reduces to the two-dimensional van der Waals equation of state when no clustering occurs. The temperature-dependent interaction parameter is obtained by fitting the experimental data for 1,2-dimyristoyl phosphatidylcholine at relatively large surface areas per molecule. Good agreement with experiment is obtained for 1,2-dimyristoyl (C-14), 1,2-dipalmitoyl (C-16), 1,2-distearoyl (C-18), 1,2-diarachidoyl (C-20), and 1,2-dibehenoyl (C-22) phosphatidylcholines (lecithins) over the entire range of surface areas and temperatures.