The infrared and Raman spectra of bicyclo[3.3.0]oct-1,5-ene in the vapor and liquid phases have been recorded and analyzed. Ab initio calculations, including those with the triple-zeta (cc-pvtz) basis set, have been carried out to predict the energy differences between five different structural forms. Both the spectra and ab initio calculations show that the cis C-2v conformation is the lowest energy form. The trans C-2h structure is predicted to lie only 65 to 229 cm(-1) higher in energy, depending on whether the effects of ring flapping are considered, but spectroscopic evidence for this conformation is limited. The dihedral angle of puckering is calculated to be 26degrees, which is the same as the cyclopentene value.