1,2-Bis(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene (1a) and their derivatives, 1,2-bis(2-methyl-5-p-tolyl-3-thienyl)perfluorocyclopentene (2a) and 1,2-bis(2-methyl-5-p-tert-butylphenyl-3-thienyl); perfluorocyclopentene (3a), were found to undergo reversible photochromic reactions in the single-crystalline phase. Upon irradiation with 366 nm light the single crystals turned blue. The blue colored crystals returned to colorless by irradiation with visible light (lambda > 480 nm). The substituents at para positions of the phenyl groups did not affect the rates of photocyclization reactions both in the single-crystalline phase and in hexane. Activation energies of the photocyclization reactions were almost zero. On the other hand, activation energies as much as 5-10 kJ mol(-1) were observed in the photocycloreversion reactions in the single-crystalline phase. These values were smaller than those observed in solution, ca. 16 kJ mol(-1) Slow thermal cycloreversion reaction of the closed-ring isomer (1b) was observed above 150 degrees C. The activation energy was 139 kJ mol(-1).