Single layers of 40 mu m NaY crystals were prepared on a CaF2 support. These are sufficiently transparent to allow recording of reactants occluded in the zeolite pores by W-visible transmission spectroscopy for the first time. Spectra of 2,3-dimethyl-2-butene (DMB) and oxygen gas loaded into the zeolite revealed the true absorption profile of the DMB . O-2 charge-transfer contact complex. Visible-light-induced reaction was monitored by in situ Fourier transform infrared spectroscopy. Irradiation resulted in the same selective oxidation to 2,3-dimethyl-3-hydroperoxy-1-butene observed previously in highly scattering pellets of 1 mu m NaY particles.