Xerogels of nanocrystalline Si-modified titanias having large surface areas and superior thermal stabilities were obtained by the glycothermal method, followed by the removal of the organic phase by flash evaporation, and their physicochemical and photocatalytic properties were investigated. With the increase in the amount of Si addition, the crystallite size of the sample decreased and BET surface area increased. The zeta potential shifted to the more negative side by Si addition, which affected the adsorbability of dyes. The obtained xerogels of Si-modified titanias were effective photocatalysts for decomposition of organic dyes and exhibited higher activities compared to JRC-TIO-4. Xerogel samples were highly dispersed in aqueous solutions because the coagulation, which may occur during the drying stage in the sample preparation, was effectively prevented and this feature is one of the critical factors for the high activities of the xerogel catalysts. Their effectiveness for photocatalytic decomposition of cationic dyes was remarkable since the negative surface charge of the catalysts enhanced the adsorbability for these dyes.