Surface and adsorption structures of dioctadecyl sulfide (DOS) self-assembled monolayers (SAMs) on Au(111) were examined by scanning tunneling microscopy (STM) and high-resolution electron energy loss spectroscopy (HREELS). In comparison with alkanethiol or dialkyl disulfide SAMs, the molecularly resolved STM images clearly showed that DOS SAMs have significant structural differences in the formation of depressions and ordered domains, and in the lattice structure, including the presence of conformational defects in the ordered domains. In addition, the clear red shift of the Au-S stretching mode and the existence of the C-S stretching mode were also observed for DOS SAMs by HREELS measurements. The unique surface structure and the existence of such characteristic features at the Au-S and C-S stretching modes for DOS SAMs strongly support the intact adsorption of DOS molecules on Au(111).