The cis-trans photoisomerization of crystalline or powdered cis,cis-1,4-diphenyl-1,3-butadiene (cc-DPB) was studied at room temperature. The progress of the reaction was monitored by fluorescence spectroscopy, powder X-ray diffraction, 1H NMR and HPLC. High conversions (up to 90%) to the trans,trans isomer were observed in a crystal to crystal reaction. Formation of the cis,trans isomer, the sole product obtained in solution and in very viscous glassy media at 77 K is entirely suppressed in the solid state. The observed two-bond photoisomerization is explained by Warshel's bicycle-pedal photoisomerization mechanism (BP). The results are consistent with X-ray diffraction measurements, which have revealed that cc-DPB molecules exist in crystals in edge to face alternating arrays of two conformer structures whose phenyl rings deviate significantly from the plane of the central diene moiety ( approximately 40 degrees ). One of the conformers has the two phenyls in parallel planes and the other in roughly perpendicular planes. Least motion considerations suggest that the former should undergo the two-bond photoisomerization more easily, in agreement with observations that indicate that the reaction proceeds in discrete stages. Recently reported cis,cis- to trans,trans-muconate photoisomerizations in the solid state are proposed to also proceed via the BP mechanism. The reactions are consistent with the X-ray crystal structures of the cis,cis-muconate isomers.