The visible light-induced reaction between 2,3-dimethyl-2-butene and O-2 in zeolite NaY at -100 degrees C was recorded on the millisecond time scale by rapid scan FT-infrared spectroscopy. The experiment revealed the growth of the sole product, 2,3-dimethyl-3-hydroperoxy-1-butene, during a 150 ms photolysis pulse. Since this duration is short compared to the time scale estimated for cage-to-cage diffusion of the product, it allowed us to determine the number of cages participating in the reaction induced by the pulse. On the basis of this measurement and a comparison of product yields in pulsed and continuous irradiation experiments, we estimate that at a minimum 1% of all NaY supercages participate in the photoreaction. This is 1-3 orders of magnitude larger than the concentration of any defect sites (Lewis acid, radical sites) that might be present in NaY zeolite. This confirms that the sole physical property of the zeolite cage, namely the high electrostatic field in the vicinity of the alkali ions, is responsible for the visible light-induced olefin oxidation.