1J. Travert, O. Saur, A. Janin,and J. C. Lavalley, J. Mol. Struct., 1976, 33, 265; O. Saur, J. Travert, J.-C. Lavalley, and M. Chabanel, C.R. Acad. Sci. Paris, 1980, c, 291, 227. 2I.W. May and E.L. Pace, Spectrochim. Acta, 1968, 24A, 1605.Vibrational data for C-deuterated methanethiols are now available1 to supplement those previously reported for CH3SH and CH3SD.2 In addition 13CH3SH(90^{13}C)hasbeenprepared,anditsvapor−phaseinfraredspectrumobtained.Onthebasisofthisinformation,anewnormalcoordinateanalysisofmethanethiolwasundertaken.Thecalculationsusedamodifiedvalenceforcefieldinwhicheightdiagonal(thetorsionalmodewasneglected)andfourinteractionconstantsweresimultaneouslyrefined.This12parameterforcefieldreproduced50observedfrequencieswithisaverageerroroflessthan9 cm^{-1}(<0.8CD_{3}$ and CSH deformations of the C-deuterated species