Resonance Raman spectra 4a,4b-dihydrophenanthrene have been obtained by irradiating degassed samples of cis-stilbene in cyclohexane and in methanol at 266 nm and subsequently exciting the Raman spectrum within the absorption band of the dihydrophenanthrene product at 436 and 477 nm. We believe that these are the first published vibrational spectra of this species. The Raman spectra at both wavelengths are dominated by a strong band at 1474 cm −1 which is presumably the mostly in-phase stretch of the double bonds in the conjugated polyene chain, shifted down in frequency by about 100 cm −1 from the corresponding all-trans linear polyene. Other weaker vibrations are observed at 769, 1035, 1080, 1147, 1201, 1298, 1534, and 1555 cm −1, and some assignments are suggested based on comparison with QCFF/PI calculations. The nonresonantly enhanced Raman lines of the other cis-stilbene photoproduct, trans-stilbene, relative to dihydrophenanthrene are much stronger in methanol than in cyclohexane under the same irradiation conditions. This observation, together with the results of simple laser photolysis experiments in which the 436 nm absorbance is used to monitor the rates of dihydrophenanthrene formation and destruction, indicate a large solvent effect on the quantum yield for photocyclization of cis-stilbene and little or no solvent effect on the yield for photochemical ring opening of dihydrophenanthrene. The ground state resonance Raman intensities of cis-stilbene excited at 266 nm indicate slightly faster distortion of the vertically excited state along torsional coordinates in methanol relative to cyclohexane solution.