Two electron correlation theories, second-order Moller-Plesset perturbation (MP2), and density functional (DFT) methods have been adopted to obtain fully optimized structures of styrene, trans-stilbene, and cis-stilbene; Full geometry optimizations with MP2 shows that the nonplanar conformations of styrene and trans-stilbene are preferred by 0.24 (styrene) and 0.80 kcal/mol (trans-stilbene), respectively. However, B3LYP, BLYP, and BVWN prefer a planar conformation contradicting the MP2 results. Due to the disorder of the crystal, X-ray experimental data of C=C double bond length of trans-stilbene seem to be too short. Vibrational spectra of these molecules are calculated at the BLYP/6-3++G** level without any empirical scaling. The agreement with experiment is excellent, some normal modes are reassigned. The dependence of the IR spectrum as a function of conformation in the 700-800 cm(-1) region allows the determination of the solution-phase conformation. Both styrene and trans-stilbene are planar in solution, implying that these molecular conformations are mainly determined by the intermolecular forces rather than intramolecular ones.