A surface thermodynamic analysis was applied to the experimentally measured pressure-area isotherms of mixed monolayer films of phosphocholine (PC) and phosphoglycerol (PG) lipids in order to quantitatively describe the phase miscibility of these physiologically important binary mixtures. We have calculated the excess free energies of mixing (ΔGxsπ) of binary mixtures of acyl chain saturated PC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine, DPPC) with saturated PG (1,2-dipalmitoyl-sn-glycero-3-phosphoglycerol, DPPG), as well as saturated PC (DPPC) with acyl chain unsaturated PG (1,2-dioleoyl-sn-glycero-3-phosphoglycerol, DOPG) at various mole fractions and under different subphase Ca2+ ion conditions. ΔGxsπ for the binary mixture of DPPC + DPPG on a NaCl subphase indicates that this binary system approximates an ideal mixture at almost all mole fractions studied. The presence of 5 mM CaCl2 in the subphase results in negative values of ΔGxsπ for the DPPC/DPPG mixture, indicating a miscible, non-ideal monolayer film, where increased intermolecular interactions are stabilized in the presence of Ca2+. The mixture of DPPC and DOPG on NaCl was found to have positive deviations from ideality at all mole fractions studied, indicating a phase separated binary mixture. The magnitude of the phase separation increased as the surface pressure increased. The presence of Ca2+ ions in the subphase was found to have little effect on the mixing of the DPPC and DOPG binary monolayer. This system remained phase separated at all mole fractions in the presence of Ca2+, with ΔGxsπ values increasing as the isotherm surface pressure increased. © 1996.