Normal coordinate calculations were carried out for the following dialkyl disulfides: dimethyl, t-butyl methyl, di- t-butyl, ethyl methyl and diethyl. Modified Urey—Bradley force constants were refined by the method of least squares to give best agreement with the observed frequencies. The functional dependence of the normal frequencies on the dihedral angles about the SS and CS bonds was calculated. It was found that the SS stretching frequency varies little with the dihedral angle about the SS bond but shows a definite dependence on the dihedral angles about the CS bonds. The latter results from a weak coupling of the SS stretching mode with the CCS bending modes. The present study confirms the correlation between the SS stretching frequency and the conformation about the CS bonds recently proposed by Sugeta, Go and Miyazawa.