A novel thermodynamic approach based on excluded area is suggested for deriving an equation of state for a surface monolayer. The article presents a general master equation accounting for the concentration dependence of the excluded area, whose particular results are the classical equations by van Laar and Frumkin as the zero approximation, the 2D van der Waals equation as the first approximation, and a new more precise state equation as the second approximation. The equation of state obtained also accounts for the effect of molecular orientation in a surface monolayer and allows the explanation of the role of molecular orientation in 2D first-order and second-order phase transitions.