The adsorption structures of alkanethiol self-assembled monolayers (SAMs) on Au were studied for various alkyl chain lengths [CH3(CH2)(n-1)SH,where n = 2, 4, 6, 8, 10, 11, 12, 14, 15, 16, and 18] in the frequency range from 150 to 3500 cm(-1) by means of high-resolution electron-energy-loss spectroscopy (HREELS). In the spectra, we observed a clear odd-even effect depending on the alkyl carbon number, which confirms that the signals were obtained from well-ordered domains of alkanethiol SAW Thus, the multiple Au-S stretching modes observed for several alkanethiol SAMs correspond to multiple adsorption sites in the ordered alkanethiol SAMs on Au(111). The variety of the adsorption sites depending on the alkyl chain length implies that the adsorption sites in alkanethiol SAMs are not simple as generally believed, but instead are complicated.