The layered structure of the LB films of merocyanine (MS)-arachidic acid (C-20)-n-octadecane (AL(18)) ternary system has been studied by means of the X-ray diffraction technique. Besides the homogeneously-stacked films of ternary system, heterogeneously-stacked films composed of alternate stacking of bilayers of the mixed system and the pure C-20 were fabricated for comparison. Two different regimes are recognized in the X-ray diffraction profiles of the homogeneously-stacked films, indicating that the MS-MS and the C-20-C-20 bilayer unit cells are responsible for the lower-order and the higher-order peaks, respectively.