Langmuir-Blodgett (LB) films consisting of an amphiphilic azobenzene (Az) polymer having a deuterated poly(vinyl alcohol) were prepared at varied lateral Az packing density, and their initial and light-modulated structures were evaluated by X-ray reflectometry and Fourier-transform infrared (FTIR) spectroscopy. X-ray reflectometry indicated that the layer structure of the LB film is strongly dependent on the lateral packing density. The more densely packed film was characterized by the better-defined structure in terms of the order of the local lamellar patterns and surface smoothness of the total film. The overall film thickness decreased and increased upon exposure to UV and visible light, respectively; however, the lamellar structure was irreversibly disordered by the first exposure to UV light. The deuterated PVA chain was found to have no appreciable orientational preference in the inplane direction, as proven by polarized transmission FT-IR, although a nematic liquid crystal (LC) placed on these LB films exhibited a definitive homogeneous in-plane alignment depending on the Az packing density [Seki, T.; et al. Thin Solid Films 1994, 243, 675]. UV light (365 nm) irradiation on the LB films induced a tilt of the alkyl part in the Az side chain from the surface normal. The time course observation of the UV light induced changes in the IR signals revealed that the motion of the alkyl part in the most densely packed LB film was retarded from that of the Az aromatic rings. The IR signals from the alkyl part reverted to the original state on subsequent visible light (436 nm) illumination, but reproducibility of the signals from the aromatic ring was poor. Finally, an increase in the Az packing density substantially retarded the response of the UV light induced homeotropic --> planar alignment change of a nematic LC placed on these Az LB films. The retarding effect was more manifest in this intermolecularly driven system.