The photoreactions of molecular complexes composed of O-3 and three 5-membered heterocyclic compounds, tetrahydrothiophene (THT), pyrrolidine (PyD), and thiazolidine (TAD), are systematically investigated using matrix-isolation infrared (IR) and UV-visible spectroscopies. Two visible-light absorption bands appear in the visible spectra obtained for O-3-THT and O-3 -PyD, whereas four bands are observed for O-3-TAD, which contains both N and S atoms in the heterocyclic ring. Upon visible-light irradiation, O-3-THT and O-3-PyD form their corresponding oxide derivatives, tetrahydrothiophene-l-oxide and pyrrolidine-N-oxide. Although two O-3-TAD complexes with different photoreactivities are detected, both structures form thiazolidine-1-oxide upon combining with O and S atom in the heterocyclic ring, but not thiazolidine-N-oxide. The mechanism of formation of these oxide compounds can be explained by the stability of the oxide compound in the triplet state formed via the combination of O(P-3) and the paired ring molecule.