The behaviour of monolayers containing dipalmitoyl phosphatidylcholine and ethyl palmitate and their mixtures at different molar fraction, using surface pressure-molecular area results, was investigated. The negative deviation from additivity of the mean molecular areas as a function of the mixture composition indicates the miscibility. The miscibility was confirmed by applying the two-dimensional phase rule, since the collapse pressure values vary with the composition of the mixtures. Also the free energy of mixing Delta G(mix) and the excess free energy of mixing Delta G(mix)(E) were determined. The negative values of Delta G(mix) and Delta G(mix)(E) indicate that the mixed monolayers are thermodynamically more stable compared to the pure ones and that the compounds in the two dimensional state experience mainly attractive interactions. (c) 2004 Published by Elsevier B.V.