A statistical-thermodynamic approach to derive an equation of state for a surface monolayer was analyzed on the basis of excluded area. The equations of state for a surface monolayer were generated by deriving a master differential equation relating surface pressure to excluded area. It was shown that the crudest solution of the master equation reproduces the known van Laar and Frumkin equations of state. For monolayers containing anisometric particles, an orientation equation of state was proposed.