Abstract Infrared spectra of methylvinylsulfide (I) has been studied between 400–3300 cm −1 for vapour, liquid, solution and solid phases. The Raman spectrum and the polarization properties have also been measured. A vibrational assignment is suggested. By measurement of the temperature dependence of IR-absorption, I is shown to exist in 2 conformers. The s- cis-form is the more stable one.