Abstract: Singlet oxygenation of 6-hydroxy-1, 4-cis, cis-cyclooctadiene (Ⅰ) which is formed in the photosensitized oxygenation of 1, 5-cis, cis-cyclooctadiene by sodium lamp illumination is described.Thermal decomposation of triphenylphosphite ozonide(Ⅱ) generate efficiently singlet oxygen, the yield of singlet oxygen increase from 70% to 98% dependence on the procedure used(Ⅰ) reacts readily with singlet oxygen generated by thermal decomposation of (Ⅱ) via abstraction of hydrogen atom from the doubly activited methylenic group at quasi-axial position, then predominantly undergoes “ene”reaction to give reduced product 5, 8-dihydroxy-1, 3-cis, cis-cyclooctadiene (Ⅲ) and 6-hydroxy-4-cis-cyclooctenone (Ⅳ) after thermal rearrangement of (Ⅲ), their spectral data are identical with literature.This result can be explained by both the existence of comformation favorable for concerted "ene" reaction and the existence of strengthened π-π intermolelar interaction in compound (Ⅰ). π⁴s + π²S cycloaddition is suppressed due to unfavorable molecular orbital interaction.