Self-assembly of n-dioctadecyl sulfide (ODS) on Au(111) has been closely investigated by using X-ray photoelectron spectroscopy (XPS), in which the binding condition of sulfur on Au(111) was determined by the S(2p) XPS peak position, and the surface density and chain conformation of adsorbed molecules were determined by the relative XPS peak intensity, C(1s)/S(Pp). The surface reaction of ODS on Au(111) was unstable unlike ODT SAM, and it was changed drastically by small variation of adsorption condition. When adsorption was carried out in 1 mM CH2Cl2 solution at room temperature, ODS molecules mostly formed fully adsorbed SAMs, intact without C-S cleavage. This was evaluated by the C(1s)/S(2p) intensity, which was twice as strong as ODT SAM, and by the S(2p) peak which appeared as a doublet at the position of "unbound" sulfur [S(2p(3/2)) at similar to 163 eV], suggesting "physisorption" of ODS on Au(111). On the other hand, when a different condition for SAM formation was used (e.g., high temperature, long time immersion, or CHCl3 as a solvent), the C(1s)/S(2p) intensity decreased to a value smaller than ODT SAM, and the S(2p) peak was shifted to lower binding energies, the "bound" (162 eV) and "free" (161 eV) sulfur positions. In these SAMs, different surface reactions including carbon-sulfur (C-S) bond cleavage seem to occur rather than nondestructive adsorption. High-resolution atomic force microscope images revealed that ODS SAM, prepared by 24-h immersion in 1 mM CH2Cl2 solution at room temperature, formed a hexagonal lattice with the lattice constant, d = 0.46 nm, which is nearly equal to the close-packed distance between alkyl chains and totally incommensurate against gold adlattice. Our data suggest a unique self-assembling process of ODS SAM, in which the chain-chain interaction is expected to be more predominant rather than the molecule-substrate interaction unlike ODT SAM.