Upon loading of 2,3-dimethyl-2-butene (DMB) and O-2 into zeolite NaY, photochemistry was observed at wavelengths as long as 760 nm. Similarly, photoexcitation of trans- or cis-2-butene and O-2 in this zeolite resulted in chemical reaction at a threshold wavelength of 600 nm. Reactions were initiated either with filtered tungsten-source light or the emission of a CW dye laser and typically conducted at -50 degrees C. Products identified by FT-infrared spectroscopy were 2,3-dimethyl-3-hydroperoxy-1-butene (>90%) and acetone in the case of DMB + O-2. trans- or cis-2-butene + O-2 gave exclusively 3-hydroperoxy-1-butene. This constitutes the first synthesis of this hydroperoxide by direct photolysis of 2-butene O-2 pairs. Laser reaction excitation spectra in the 500-700-nm region revealed a continuous absorption for both the DMB.O-2 and the 2-butene.O-2 systems. It is attributed to a charge-transfer transition. Comparison with corresponding absorption spectra in conventional media shows that the excited alkene.O-2 charge-transfer states are stabilized by electrostatic interactions with the zeolite NaY environment by 12 000 cm(-1). Substantially less stabilization was observed in high-silica faujasite. Thermal decomposition of the hydroperoxide photoproducts upon warm-up from -50 degrees C to room temperature was monitored by infrared spectroscopy. It was found that 3-hydroperoxy-1-butene thermally epoxidizes the excess cis- or trans-2-butene in room temperature NaY under complete stereochemical retention.