Zirconia-supported Ti-substituted Keggin-type polyoxometalates, Li5PW11TiO40/ZrO2 (PW11Ti/ZrO2) and K7PW10Ti2O40/ZrO2 (PW10Ti2/ZrO2), were prepared by incorporating PW11Ti and PW10Ti2 cluster into a zirconia matrix via a sol-gel technique. These insoluble and readily separable composites were characterized by DR-UV (DR: diffuse reflectance) and FT-IR spectra, P-31 MAS NMR, ICP-AES, and nitrogen adsorption determination, indicating that the clusters were chemically attached to the zirconia supports, and the primary Keggin structure remained intact. The photocatalytic activity of the supported PW11Ti and PW10Ti2 was tested via degradation of an aqueous dye naphthol blue black (NBB). It indicated that the dye NBB can be degraded totally and mineralized into the inorganic products such as CO2, NH4+, NO3-, and SO42- ions by irradiating the composite slurry in the near-UV area. Dropped of PW11Ti or PW10Ti2 cluster from the zirconia matrix into the reaction system was hardly observed during the photocatalytic tests, attributed to the strong chemical interactions between the Keggin units and the zirconia support. (C) 2003 Elsevier Ltd. All rights reserved.