irradiation of &methyl; spiro [2.5] oct-7-en-4-one through its singlet excited state resulted in the formation of 4-methyl spiro [2.5] oct-4-en -6-one. we have used the cyclopropyl moiety in the molecule as a probe to study the operating mechanism in the photochemistry of ?, ?-unsaturated compounds. the results showed that this photoisomerization probably occurs through a concerted process.