A recently developed equation of state (Feng, S.-S.; MacDonald, R. C. Biophys. J. 1995, 69, 460) fo:r lipid monolayers at the air/water or oil/water interface was employed to fit experimental pi-A curves of various lipids with different chain number, chain length, and chain unsaturation. In this kind of equation of state the distributional entropy of the lipid headgroups within the membrane plane and the conformational energy of the lipid chains of various unsaturation can be exactly calculated. The three parameters :in the equation of state, namely, a(m)/a(0), ratio of the minimal molecular surface area to the cross-sectional area of the two chains of a single lipid molecule, gamma(b), the ratio of the effective to the actual projected surface area increment, and pi(3), the residual part in the total surface pressure, were determined and used to reproduce the experimental data. Such an equation of state can accurately describe the properties of most lipid monolayers in their liquid-expanded state. To overcome the unreliability of experimental data at the collapse pressure, the interfacial elastic modulus of monolayers at their collapse pressure was obtained by extrapolating the reproduced pi-A curves to the collapse pressure rather than directly from experimental pi-A isotherms. The results indicate that the interfacial elastic modulus increases with the number of double bonds until a peak is reached at the second or the third double bond, corresponding to a peak in the minimal molecular area. Furthermore, the interfacial elastic modulus is lowered if the double bonds are located in the middle of the hydrocarbon chains. For liquid-expanded monolayers of the same degree of unsaturation, the elastic modulus decreases with chain length. A comparison between the interfacial elastic modulus of lipid monolayers at the collapse pressure and that of the same lipid bilayers at their tension free state strongly supports the assertion that a lipid monolayer at its collapse pressure mechanochemically corresponds to a leaflet of large bilayer vesicles of the same lipid in water, which is developed from the monolayer-bilayer correspondence theory (Feng, S.-S. Langmuir 1999, 15, 998).