We present preliminary results of an experimental investigation of ordered phases of CH3S chemisorbed on Au(111). The self-assembled monolayer has been grown by dosing dimethyl disulfide in ultrahigh vacuum at different substrate temperatures between 200 K and 320 K and following different protocols. The monolayers have been characterized by means of low-energy He atom scattering with time-of-flight detection in a temperature range between 150 K and 300 K. The observed diffraction patterns show that the main periodicity is well-described by the (3 x 4) overlayer of the Au(111) lattice, coexisting with the (root 3 x root 3) periodicity, in agreement with previous results obtained by means of low-energy electron diffraction.