The surface structures, adsorption conditions, and thermal desorption behaviors of benzenethiol (BT) and benzenemethanethiol (BMT) self-assembled monolayers (SAMs) on Au(111) were examined by means of scanning tunneling microscopy (STM), X-ray photoelectron microscopy (XPS), and thermal desorption spectroscopy to understand the effects of the alkyl spacer between the phenyl group and the sulfur atom. Although XPS spectral shapes in the S 2p region for both SAMs are similar, the Surface structures and thermal desorption behaviors differ significantly. FIT SAMs on Au(111) were composed of disordered phases, whereas BMT SAMs have well-ordered phases containing vacancy islands. The strong desorption peak for parent mass species (m/z = 110, C6H5SH+) was observed in BT SAMs at about 500 K, whereas no desorption peak (m/z = 124, C6H5CH2SH+) was observed from BMT SAMs. Interestingly, the dominant TD peak for the benzyl fragments (m/z = 91, C6H5CH2+) formed via C-S bond cleavage was observed in BMT SAMs at around 400 K. From this study, we clearly revealed that the small modification in chemical structure by inserting a methylene spacer between the phenyl group and the sulfur atom affects 2D SAM structures, adsorption conditions, and thermal desorption behaviors and stability. The results obtained here will be very useful in designing and fabricating aromatic thiol SAMs for further applications. (C) 2009 Elsevier Inc. All rights reserved.