Variable temperature (-60 to -100°C) studies of the infrared spectra of dimethyl vinyl chlorosilane, CH 2 CHSi(CH 3 ) 2 Cl, and methyl vinyl dichlorosilane, CH 2 CHSiCH 3 Cl 2 , dissolved in liquefied xenon have been recorded. For both of these molecules the cis and gauche conformers have been identified in the fluid phases but only the conformer with the methyl group eclipsing the double bond remains in the solid. From these data, the enthalpy difference for dimethyl vinyl chlorosilane has been determined to be 41 ± 9 cm - 1 (117 ± 26 cal mol - 1 ) with the gauche conformer being the more stable rotamer and, for methyl vinyl silane, the enthalpy difference has been determined to be 77 ± 8 cm - 1 (220 ± 23 cal mol - 1 ) with the cis conformer being the more stable rotamer. Ab initio calculations have been performed using the MP2/6-31G * basis set and a comparison between the calculated skeletal structural parameters for both molecules is made with those obtained for vinyl trichlorosilane as well as with some electron diffraction data.