A syn-tricyclo[4.2.0.0(2,5)]octane derivative isomerized to a cycloocta-1,5-diene derivative on exposure to X-ray radiation keeping the single crystal structure. The two half-cylindrical molecules formed a cavity, in which three cyclohexanes used as recrystallization solvent were included. The single crystal obtained from a mixture of chloroform and ether was analyzed at -140 degrees C by X-ray crystallography, which confirmed the structure of syn-tricyclo[4.2.0.0(2,5)]octane.