The phototransformation of aryl chlorides adsorbed on MFI zeolites is markedly different from that observed in solution and other solid surfaces such as silica. The formation of radical cations and the constraints imposed by the channels shift the reactivity from the C-Cl bond to the O-R bond. Irradiation generates kinetically stabilized intermediates that can be characterized using conventional steady-state spectroscopic techniques, and these intermediates can be used as ordinary chemical reagents.