The interaction of two ubiquinones having different chain lengths, ubiquinone-2 and ubiquinone-10 with 2 saturated or 10 unsaturated isoprenoid groups, with dipalmitoylphosphatidylcholine (DPPC has been studied in mixed monolayers. The miscibility of the mixtures has been investigated by pressure-area measurements and simultaneous fluorescence microscopy at the air-water interface. Results were analyzed in terms of the additivity rule and the excess free energies of mixing were calculated from the compression isotherms in the full range of ubiquinone concentrations. We have also evaluated the area contribution of each ubiquinone molecule to the total molecular area of the mixed films. Stable monolayers are formed by the oxidized forms of the two ubiquinones but by increasing the lateral pressure, the ubiquinones are expelled from the films. The difference observed between the two ubiquinones in mixed monolayers are explained by a difference of localization according to their side chain lengths.