2,4,6-Triphenylpyrylium ion (TP+), a well-known single electron transfer photosensitizer, encapsulated inside the voids of a microporous Y zeolite (TPY) (spherical cages of 1.3 nm diameter, tetrahedrally interconnected through four openings of 0.74 nm diameter) has been tested as heterogeneous photocatalyst for the degradation of 4-chlorophenoxyacetic acid as model probe molecule. The results were compared under simultaneous irradiation with the performance of TiO2, TP+BF4- supported on amorphous silica and malachite green (another dye) also imprisoned inside Y zeolite. It was found that the activity of TPY is higher than that of the other photocatalytic systems, especially using visible light excitation or direct solar irradiation. These results have been interpreted as being due to a combination of factors, including appropriate absorption spectrum of TP+, favorable thermodynamics of the electron transfer pathway and a co-operative contribution of the zeolite host. Thus, we have observed that an initial rapid step consisting in the adsorption of the pollutant inside the zeolite micropores takes place before the actual slower photoreaction enables detection of photoproducts. This adsorption produces a favorable high concentration of the target substrate in the proximity of the reactive photocatalytic center, thus increasing the efficiency of the overall degradation process.