A surface equation of state, applicable to liquid-expanded (LE) monolayers, was derived by analyzing the Helmholtz free energy of the LE monolayers. Based on this equation, a general equation was obtained to describe all states of single-component phospholipid monolayers during comprassion. To verify the applicability of the equation, pi-A isotherms of 1,2-dipalmitoylphosphatidylcholine (DPPC), 1,2-dipalmitoylphosphatidylglycerol (DPPG), and 1,2-dimyristoyphosphatildylcholine (DMPC) were measured. The comparison between model and experimental values indicates that the equation can describe the behavior of pure phospholipid monolayers.