The synthesis and photocatalytic properties of a heteropolyoxoniobate, K-10[Nb2O2(H2O)(2)][SiNb12O40]center dot 12H(2)O (1), are reported, revealing an important role of Zr4+ additives in the crystallization. Compound 1 exhibits overall photocatalytic water splitting activity, and its photocatalytic activity is significantly higher than that of Na-10[Nb2O2] [SiNb12O40]center dot xH(2)O (2). Fluorescence lifetime measurements suggest that the enhanced photocatalytic activity of 1 likely results from a larger yield of longer-lived charge trapping states in 1 due to the coordination of one water molecule to the bridging Nb5+, leading to highly unsymmetrical seven-coordinated Nb5+ sites.