The photoluminescence properties and photocatalytic reactivities of the V-HMS mesoporous zeolite have been investigated by means of in situ photoluminescence. UV-vis, EXAFS, XRD and ESR measurements. It was found that the V-HMS zeolite involves a vanadium species which is highly dispersed and incorporated in the framework ol the mesoporous zeolites having a V-O bond and a C 3v symmetry. This species plays an important role as the emitting site of this zeolite. It was also found that the zeolite exhibits photocatalytic reactivity under UV irradiation in the presence of cis-2-butene. resulting in the formation of trans-2-butene and I-butene. The photocatalytic reactivity of the V-HMS mesoporous zeolite was found to be much higher than that of the V-silicalite microporous zeolite (VS-I 1). The efticiency of the dynamic quenching of the photoluminescence of the V-oxide species in the excited state in the V-HMS mesoporous zeolite by smaller molecules such as 0, was found to be the same as that of the VS-I microporous zeolite. By comparing these results with results obtained on the VS-I microporous zeolite photocatalyst, we can conclude that the pore size effect plays a significant role as one of the major factors which determine the photocatalytic reactivities of porous zeolite photocatalysts. lr