Abstract: Evidence of the dual isomerization mechanism is given for the cis-and trans-ocimene formed from c-pinene sensitized by xanthone. The quenching rate constant of triplet excitedxanthone by cis-ocimene was determined experimently to be 1.87×10¹⁰ (mol/dm³)^-1s^-1 which is muchfaster than the quenching rate constant of the excited state by aaaaaaaaaaaaa-pinene. Therefore, the paper points out that the conversion of the isomerization reaction is limited by the preferential quenching process of the products.