Sodium, niobium, and tantalum phosphate bronzes Na4M8P4O32 (M=Nb, Ta) are employed as photocatalysts for water splitting to reveal the effects of the distortion of metal–oxygen octahedra on the photocatalytic performance. Addition of RuO2 as a co-catalyst leads to high, stable activity in the stoichiometric production of H2 and O2 under UV irradiation. The combination of highly crystallized phosphates and a high dispersion of RuO2 particles result in high photocatalytic activity. The sodium niobium phosphate bronze Na2Nb8P4O32, consisting of a framework built up from slabs of corner-sharing NbO6 octahedra connected through isolated PO4 tetrahedra, provide heavily distorted NbO6 octahedra with large internal dipole moments. The results support the existing view that the activity correlates with the magnitude of the dipole moment. The heavy distortion of NbO6 octahedra is shown to play a significant role in photocatalytic water splitting.