1,2-Bis(2-ethyl-5-phenyl-3-thienyl)perfluorocyclopentene (1a) was found to undergo a thermally reversible photochromic reaction in solution as well as in the single-crystalline phase. Upon irradiation with ultraviolet light the hexane or toluene solution containing la and single-crystal la turned blue along with the formation of closed-ring isomer (Ib). The blue solution and single crystal returned colorless by irradiation with visible light (lambda > 500 nm) or heating above 100 degreesC. The bleaching is due to the cycloreversion of Ib to la. The thermal cycloreversion activation energies were measured in the toluene solution and in the single-crystalline phase above 100 degreesC. The activation energy from photogenerated Ib to la in the solution was 128 kJ mol(-1), while it decreased to 120 kJ mol(-1) in the crystal of the open-ring isomer la. In the crystal of the closed-ring isomer Ib the energy increased to 137 kJ mol(-1). The cycloreversion reaction was found to be controlled by the environmental conditions. The thermal cycloreversion reaction in the crystal was directly followed by X-ray crystallography and revealed to proceed in a conrotatory mode, which is opposed to the general Woodward-Hoffmann rules.