Dialkyl sulfide self-assembled monolayers on gold were characterized by Fourier transform infrared reflective absorption spectroscopy (FTIR-RAS) and scanning tunneling microscopy (STM). It was found that dialkyl sulfide SAMs have quite different surface characteristics compared to alkanethiol SAMs. Our STM result clearly elucidated that dialkyl sulfide SAMs are composed of the liquid-like disorder phase with many small nucleations, not the ordered phase.