Experimental findings show that ethylene included in a zeolite cavity absorbs visible light to allow photochemical reactions. This evident supramolecular effect is explained in this Letter in terms of a charge transfer transition upon excitation. According a priori NDOL calculations the charge transfer of ethylene occurs between the HOMO, localized on the trapped molecule, and electronic states distributed on the zeolite network, together with a very perturbed intramolecular transition in ethylene. The cases of methane and ethane were also studied and a drop of about 4 eV is always observed in the first electronic transition energy when included in the zeolite cage with respect to the isolated molecules. (C) 2002 Published by Elsevier Science B.V.