Cis- andtrans-azobenzenes adsorbed in NaY, Na-mordenite, and Na-ZSM-5 were irradiated using 313- and 254-nm lights. Thecis-transisomer ratio in the photostationary state produced in the zeolites was compared with that in cyclohexane. It was found that Na+in the cavities did not significantly affect the ratio although for stilbene a drastic change in the ratio was caused most likely by electrostatic interaction between the carbon-carbon double bond and Na+. For azobenzene it seems probable on the basis of the results obtained by semiempirical molecular orbital calculation (AM1) that Na+interacts with a lone pair of one of the two nitrogen atoms, but not with the π-orbital of the nitrogen-nitrogen double bond.