9,10-Dicyanoanthracene (DCA)-sensitized photooxidation of trans,trans-1,4-diphenyl-1,3-butadiene (DPB), trans-stilbene (TS), and alpha-pinene (PE) incorporated in Nafion membranes has been investigated. While the oxidation in homogeneous solution yields the products derived from both the energy transfer and the electron transfer pathways, that within Nafion membranes selectively gives either the singlet oxygen mediated or the superoxide radical anion mediated products depending on the status and location of the substrate and sensitizer molecules in the reaction media. Upon incorporating the alkene within the water-swollen Nafion membranes and dissolving DCA in the surrounding CH2Cl2 solution, the sensitizer is isolated from the substrate during the irradiation time and the electron transfer between them is prevented. The singlet oxygen produced in the surrounding solution diffuses into the Nafion clusters and reacts with the alkenes. Thus, only the singlet oxygen oxidation products are obtained and no product derived from superoxide radical anion is detected. In contrast, incorporating both the substrate and the sensitizer within the water-swollen Nafion membranes leads to the possibility of situating the two dissimilar molecules near one another within the restricted space of the Nafion cluster. Thus, the electron transfer from the alkenes to the singlet excited state of DCA is enhanced, and the efficiency of the intersystem crossing from (1)DCA* to (3)DCA* is reduced. The photosensitized oxidation in this case only gives products derived from the electron-transfer pathway, and no singlet oxygen mediated product is observed.