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Incorporation of organic molecules inside the micropores of zeolites may produce significant changes on the molecular properties of the adsorbed guests. 1,2 Some of these changes such as an enhanced basicity and an apparent decrease of the oxidation potential are particularly remarkable when a host/guest tight fit occurs. 3 These variations cannot be easily rationalized by the current paradigms, one of the most widely invoked being the influence of the zeolite intense electrostatic fields on the stabilization of polar adsorbates. 4-6 An electronic confinement concept has been proposed based on the concept that the molecular orbitals (MO) of the adsorbates inside the zeolite pores are not extended over all the space, as they are in the gas phase, but instead are forced to limit within the zeolite walls. 7,8 This "boxing effect" is stronger as the size of the confined guest approaches the zeolite cage dimension, producing an energy increase of all MO particularly those which are more diffuse. The HOMO has been predicted to be more sensitive than the LUMO, and the predicted effect is a reduction on the band gap of the frontier orbitals that has been interpreted as a preactivation of the molecule inside the zeolite pores compared to gas phase. 7,9 This electronic confinement should be also reflected in the optical properties of the guest.No experimental evidence in support of this concept has been given apart from indirect qualitative reactivity tests. In the present contribution we show convincingly that the predictions of this electronic confinement theory are indeed realized. Anthracene (AN) was found to be a suitable probe molecule for this purpose since it is a conformationally rigid molecule having large emission quantum yields at adequate wavelengths and similar equilibrium geometries in S 0 and S 1 . Several reports on the photophysical properties of AN in zeolites have been published, but none of them have paid attention to the shifts in its electronic properties upon incorporation. 10,11 Low loadings of AN (5 mg‚g -1 ) were adsorbed within three zeolites in their neutral Na + -form with different geometries and pore dimensions, that is, Y (spherical supercages of 1.3 nm diameter tetrahedrally interconnected through 0.74 nm windows), mordenite (elliptical channels 0.65 × 0.70 nm 2 ), ZSM-5 (array of straight channels 0.51 × 0.55 nm 2 intersecting perpendicularly with sinusoidal channels 0.52 × 0.56 nm 2 ). Incorporation of AN was carried out by stirring at room temperature a CH 2 Cl 2 suspension (6 × 10 -6 ML -1 ) in the presence of dehydrated zeolites (aerated calcination at 550°C for 6 h) for 1 h. Then, the solid was filtered and washed with fresh CH 2 Cl 2 . XPS analysis combined with Ar + sputtering indicates that the C/Si atomic ratio increases from 0.03 to 0.14, going from the external surface to ∼100 Å down. The formation of aggregates or AN •+ radical cation reported using acid zeolites under other conditions was not observed here at our loadings using the neutral Na + form. Figure 1 shows the cor...