Zirconium-silicon binary oxide catalysts having different Zr concentrations were prepared by the sol-gel method and used as photocatalysts. The photocatalytic activity of these catalysts was investigated for the isomerization of cis-2-butene and was found to be enhanced in regions of lower Zr content. In situ photoluminescence, UV-vis reflectance, XRD, EXAFS, and XPS spectroscopic investigations of these zirconium-silicon oxide catalysts indicated that the Zr ions in the oxides which have a low Zr concentration exist separately as extremely small zirconium oxide species in the SiO2 matrix while the radiative decay from the charge-transfer excited state of these species results in a characteristic photoluminescence. These results and the quenching of the photoluminescence by the added butene molecules showed that the higher photocatalytic activity of the zirconium-silicon binary oxides having a low Zr content is linked to the catalytic activity of the charge-transfer excited state of the zirconium oxide species, [Zr3+-O-]*, in the catalysts.