The adsorption characteristics of a Langmuir-Blodgett (LB) film of octadecylamine (ODA) transferred from the surface of an alkaline subphase were investigated by employing dye material of methyl orange (MO) as an adsorbate, As a result, specific adsorbability of the ODA LB film was observed, similarly to that of the LB film of long-chain quaternary alkylammonium salt. The structural characterizations of the ODA LB films before and after MO adsorption were also carried out by applying spectroscopic methods. It was found that the MO molecules took the specific molecular arrangements of edge-on orientation on the LB him and that the bilayer in the LB films had an interdigitated structure. Furthermore, the layer-layer distance of the LB film was fairly decreased by the MO adsorption, indicating the formation of another layer structure.