We studied adsorption structures and adsorption states of COOH-terminated alkanethiols with different alkyl chain lengths in self-assembled monolayers (SAMs) by X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), and near-edge X-ray absorption finestructure (NEXAFS) spectroscopy. In the SAM of 11-mercapto-undecanoic acid on an Au(1 1 1) Surface, formation of a well-ordered monolayer was confirmed from the results of the S 2p XPS and STM observation, while this was not the case in the SAM of 6-mercapto-haxanoic acid. C K-edge NEXAFS results suggested that the orientation or conformation of the alkyl chain differs between two SAMs. These results indicate that the intermolecular interaction of the alkyl chains plays an important role for the formation of a well-ordered monolayer of COOH-terminated alkanethiols on Au(1 1 1). (c) 2004 Elsevier B.V. All rights reserved.