Irradiation of solutions of aromatic aldehydes or ketones generally results in photoinduced hydrogen abstraction and yields mixtures of both asymmetric and symmetric coupling products of the radicals resulting from hydrogen abstraction. In contrast, irradiation of hydrocarbon—faujasite zeolite slurries of aromatic aldehydes or ketones results in production of radical pairs by intermolecular hydrogen abstraction, which is followed by the formation of good yields of the asymmetric geminate coupling products. The mechanistic basis of this difference in chemoselectivity is proposed to result from a combination of strong preferential adsorption of the carbonyl compounds to the internal surface of the zeolite and inhibition of the diffusional motion of the geminate radical pair produced by photolysis by the “spectator” hydrocarbon molecules that occupy the internal surface but which do not participate chemically in the reactions.