Ab initio SCF MO calculations at the 3–21G level and the vibrational spectra [Raman and Fourier transform (FT) IR] of liquid-phase 2-chloro- and 3-chlorostyrene are reported. For 2-chlorostyrene, the ab initio calculations predict skew and gauche minima, with an energy difference of 11.5 kJ mol−1, and no evidence of more than one conformer was found in the liquid-phase Raman and FTIR spectra. The Raman spectra of liquid 3-chlorostyrene show pairs of bands whose temperature-dependent intensities clearly suggest the occurrence of a conformational equilibrium. From these bands, assigned to non-planar cis and trans conformers, a trans–cis energy difference of 2.3 ± 0.5 kJ mol−1 was obtained for the liquid phase.