A hybrid monolayer film of Au nanoparticles, half-covered with dioctadecyldimethylammonium chloride (DODAC), was prepared at the air/water interface and characterized using, transmission electron microscopy (TEM), a quartz-crystal microbalance, and infrared spectra measurements. TEM images of the hybrid film showed that the distribution of An nanoparticles depends on the surface density of DODAC and reaction time. IR spectral data provided evidence for a surface-enhanced effect of the An nanoparticles. The wavenumber of CH2-stretch vibrations of DODAC in the infrared external reflection spectra revealed that the DODAC molecules were adsorbed onto the An nanoparticles in a close-packed crystalline state for any surface density of DODAC, which is different front the usual behavior of Langmuir monolayers. (c) 2004 Elsevier Inc. All rights reserved.