We report a study of the structure of phenyloxirane (1) in solution using ab initio vibrational spectroscopy. Vibrational unpolarized absorption and circular dichroism spectra of 1 in CCl4 and CS2 solutions are measured. Harmonic potential energy surfaces and, thence, harmonic frequencies, dipole strengths, and rotational strengths of the fundamental modes of 1 are calculated ab initio using Hartree-Fock/self-consistent field (HF/SCF). MP2 and density functional theory (DFT) methods. DFT/B3LYP/TZ2P vibrational spectra an in excellent agreement with experimental spectra; alternative combinations of functional and basis set yield less good agreement. HF/SCF and MP2 calculations yield spectra in even poorer agreement with experiment. The DFT/B3LYP/TZ2P equilibrium structure is therefore the most accurate. The plane of the phenyl ring is tilted from perpendicular to the plane of the oxirane ring by 14.5 degrees. The direction of tilt brings a phenyl ortho H closer to the oxirane O. The direction and magnitude of tilt can be understood in terms of the competing forces of pseudoconjugation, C-H ... O hydrogen bonding and H ... H van der Waals repulsion. The DFT/B3LYP/TZ2P dipole moment agrees with experiment within experimental error.