The photochemical reactions of azobenzene (AB) incorporated into different solvent-swollen Nafion membranes were investigated. The location of azobenzene in water-swollen Nafion membranes was different from that in methanol-swollen Nafion membranes, which was responsible for the different photochemical reactions of AB. In methanol-swollen Nafion membranes only trans-cis isomerization took place, while in water-swollen Nafion membranes photochemical cyclization of AB occurred after rapid trons-cis isomerization. The relation between the relative quantum yields and the light intensity showed that the cyclization was a two-photo process. The numbers of AB molecules per cluster (nAB /cluster) in Nafion membranes with different AB concentrations and different water contents were calculated. Under the experimental condition two competitive photocyclization mechanisms were suggested by nAB /cluster in Nafion membranes, relative quantum yields of the cyclization, and the change of the ratio of benzidine to benz The photochemical reactions of azobenzene (AB) incorporated into different solvent-swollen Nafion membranes were investigated. The location of azobenzene in water-swollen Nafion membranes was different from that in methanol-swollen Nafion membranes, which was responsible for the different photochemical reactions of AB In methanol-swollen Nafion membranes only trans-cis isomerization took place, while in water-swollen Nafion membranes photochemical cyclization of AB occurred after rapid trons-cis isomerization. The relation between the relative quantum yield and the light intensity showed that the cyclization was One two-photo process. The numbers of AB molecules per cluster (nAB / cluster) in Nafion membranes with different AB concentrations and different water contents were calculated. Under the experimental condition two competitive photocyclization mechanisms were suggested by nAB / cluster in Nafion membranes, relative quantum yields of the cyclization, and the change of the ratio of benzidine to benz