Cycloaddition reaction of singlet oxygen and 4-phenyl-1,2,4-triazoline-3,5-dione(PTAD) to various cycloheptatriene derivatives was investigated. The addition of PTAD to 3,8a-dihydroazulen-1(2H)-one gave exclusively a norcaradiene adduct whereas the addition to 3,4-dihydroazulen- 1(2H)-one resulted in the formation of a cycloheptatriene adduct. Photooxygenation of dihydroazulen-1(2H)-oneafforded solely a [2+ 4] cycloaddition product derived from cycloheptatriene. Photooxygenation of the reduced product, 1,2,3,8a-tetrahydroazulen-1- yl acetate gave the all possible cycloaddition products. The product distribution was not affected upon reduction of the carbonyl group. On the other hand, photooxygenation of dimethyl cyclohepta-3,5,7-triene-1,3-dicarboxylate gave mainly addition products derived from the norcaradiene structure. The formation of the products was explained by a photochemically allowed 1,7-suprafacial hydrogen shift under the reaction conditions followed by singlet oxygen addition.