Spread monolayers of dipalmitoyl phosphatidylcholine, methyl palmitate and their mixtures were studied. For this purpose, the surface pressure (π)–average molecular area (A) isotherms were recorded using a Langmuir film balance. From the π-A results information on the mutual miscibility of the two components in the two-dimensional state was deduced. Then classical surface chemistry thermodynamic analysis was performed on these isotherms, which involved calculating the excess free energy of mixing, ΔG E m ix. The negative values of ΔG E m ix indicate that the compounds in the two-dimensional state experience mainly attractive interactions.