Non-substituted 1,2-bis(3-thienyl)perfluorocyclopentene 1a was synthesized and its photochromic reactivity was compared with 1,2-bis(2,5-dimethyl-3-thienyl)perfluorocyclopentene 4a. Upon irradiation with ultraviolet light 1a easily decomposed and two kinds of by-products were produced in addition to the normal closed-ring isomer. The structures of the by-products were determined by X-ray crystallographic analysis.