Effects of electron irradiation on a methylthiolate (CH3S) self-assembled monolayer on Au(111) have been studied by using scanning tunneling microscope (STM), Auger electron spectroscopy (AES), low-energy electron diffraction, and thermal-desorption spectroscopy (TDS). AES results indicated that irradiation of the CH3S/Au(111) surface by an electron beam induces electron-stimulated desorption (ESD), in which mainly the methyl moiety desorbs via S-C bond cleavage, leaving a sulfur atom on the surface. From a quantitative analysis of TD spectra, the cross section of the ESD is estimated to be 2.8 x 10(-17) cm? for 100-eV electrons. Two-dimensional arrangements of the methylthiolate and the sulfur species under the ESD process have been investigated with STM. The STM observations indicated that the sulfur adatoms form 2D islands that grow in size with the progress of the ESD process, while unaffected methylthiolates are distributed between the sulfur islands. After the completion of the ESD, the Au(111) surface is covered with small patches (less than or similar to 100 Angstrom) of sulfur domains.