The adsorbability of the Langmuir-Blodgett (LB) film of octadecylamine (ODA) has been investigated using a-naphthol orange (NO) and methyl orange (MO) as adsorbates. It was found that the specific adsorptions of these azo dyes occurred on the ODA LB film predominantly by ionic interaction, and also that the adsorption behavior depended on pH according to the ionic states of NO and MO as well as that of the ODALB film. The structural characteristics of the LB films such as orientation, layer-layer distance, and topography of the film surface have been evaluated by means of FTIR and W-vis spectroscopies, X-ray diffraction in the low-angle region, and atomic force microscopy (AFM), respectively. The results obtained indicated that the adsorption of dyes induced rearrangements of the layer structure, where the hydrocarbon chains took an interdigitated structure and the dye molecules in the LB film exhibited remarkable distinction in the adsorbing state, i.e., a lie-flat configuration of NO and an edge-on configuration of MO.