Density functional theory (DFT) calculations on the initial charge-transfer step in the photo-oxidation of alkenes in cationic zeolites are presented. The model system used represents a part of the Y-zeolite supercage containing two II sites with alkali-earth cations and coadsorbed 2,3-dimethylbutene-2 (DMB) and O-2 on them. It is found that the electrostatic field of the zeolite cavity plays only a minor role for the stabilization of a charge-transfer state, whereas the relative orientation and the distance between the DMB and O-2 molecules are the most important factors. On the basis of these results, the photo-oxidation considered is due to a confinement effect in which the reagents are oriented in a suitable "pre-transition state" configuration.