The Raman and infra-red spectra of methane sulphonyl chloride and methane sulphonyl fluoride have been measured. Assignments of the vibrational frequencies of the two molecules have been made ; the frequency 377 cm.-1 is characteristic of the S-Cl linkage and 1210 cm.-1 of the S-F linkage in these compounds. An improved method for the preparation of methane sulphonyl fluoride is described.