The Raman spectra of liquid 3-methylstyrene show pairs of bands whose temperature-dependent intensities clearly suggest the occurrence of a conformational equilibrium. From these bands, assigned to cis and trans conformers, a trans-cis energy difference of 0.8 + 0.2 kJ mol−1 was obtained for the liquid phase. On the other hand, ab initio calculations at the 3-21G** and MP2/3-21G* levels predict a cis form more stable than the trans form by less than 0.15 kJ mol−1, in good agreement with gas-phase literature results.