Binary mixtures of cholesterol, ergosterol, and lanosterol with phosphatidylcholines differing in the length of the saturated acyl chains, viz 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1-palmitoyl-2-myristoyl-sn-glycero-3-phosphocholine (DMPC), were analyzed using a Langmuir balance for recording force-area (pi-A) and surface potential-area (psi-A) isotherms. A progressive disappearance of the liquid expanded-liquid condensed transition was observed in mixed monolayers with DPPC after the increase in the content of all three sterols. For fluid DMPC matrix, no modulation of the monolayer phase behavior due to the sterols was evident with the exception of lanosterol, for which a pronounced discontinuity between mole fractions of X = 0.3 and X = 0.75 was discernible in the compression isotherms. Condensing and expanding effects in force-area (pi-A) isotherms due to varying X(sterols) and differences in the monolayer physical state were assessed from the values for the interfacial compression moduli. Surface potential measurements support the notion that cholesterol and ergosterol, but not lanosterol, reduce the penetration of water into the lipid monolayers. Examination of the excess free energy of mixing revealed an enhanced stability of binary monolayers containing cholesterol compared to those with ergosterol or lanosterol; the differences are emphasized in the range of surface pressure values found in natural membranes.