The photosensitized oxidations of the olefins, trans-1,2-dimethoxystilbene (DMOS), trans-stilbene (TS), and trans,trans-1,4-diphenyl-1,3-butadiene (DPB) as well as the amine 2,2,6,6-tetramethylpiperidine (TMP) in mixed surfactant vesicles were investigated. The sensitizer was either a hydrophobic dye, tetraphenylporphyrin (TPP), or a cationic dye, methylene blue (MB). The substrate molecules were solubilized in the bilayer membranes of one set of vesicles, and the sensitizers were incorporated in the bilayers or the aqueous inner compartments of another set of vesicles. The irradiation samples wore prepared by mixing the above two sets of vesicle dispersions. Photoirradiation of the oxygen-saturated samples resulted in the oxidation of the substrates, as evidenced by the isolation of the end products in the olefin oxidation and by the detection of the ESR spectrum of the nitroxide radical in the amine oxidation. The quantum yields for the product formation were enhanced significantly in D2O dispersions compared with those in H2O medium. All of these observations suggest that singlet oxygen generated in the bilayer or the inner water pool of one vesicle is able to diffuse out and enter into the bilayer of another vesicle through the aqueous dispersion and react with the target molecules. The measurements or the quantum yields revealed a substantial fraction of the singlet oxygen diffusing from its generated locus to the reaction sites: 8% in H2O and 15% in D2O dispersions in the case of singlet oxygen generated in the inner aqueous compartment of the vesicle; 20% in H2O and 80% in D2O dispersions for the singlet oxygen generated in the bilayer of the vesicle. In the photosensitized oxidation of TS and DPB in vesicles, the 1,2-cycloaddition products of singlet oxygen to the olefins were detected in quantitative yields, which was in sharp contrast to the oxidation in homogeneous solutions where the 1,4-cycloaddition products of the singlet oxygen to the dienes were the unique products. This result indicates that the organized semirigid environment in vesicles prevents the olefins from conformation change.