Four types of bismuth oxides, Na3Bi3O8, NaBiO3, alpha-Bi2O3, and epsilon-Bi2O3, were obtained by hydrothermal reactions using NaBiO3 center dot nH(2)O in NaOH solution. The crystal structure of a new phase (Na-3 Bi3+)Bi25+O8 ((Na0.75Bi0.25)(2)BiO4) was determined by using single crystal X-ray diffraction data, and this compound was found to show a Na2MnCl4-related structure with a monodinic system (space group, Pm) with the following lattice parameters: a = 5.990 (2) angstrom, b = 3.335 (2) angstrom, c = 10.108 (2) angstrom, and beta = 91.08 (3)degrees. The final R-factors R-1 and wR(2) were 0.041 and 0.090 (all data), respectively. The new phase was composed of mixed valence states of Bi (Bi3+ and Bi5+, with a mean Bi valence of 4.30) with five distinct Bi sites, where two Bi5+ (Bil and Bi2) fully occupied the distorted octahedral sites and three Bi-5(+) (Bi3, Bi4, and Bi5) were statistically distributed at the split sites with Na+ (Na3, Na4, and Na5). The Na6 site is fully occupied. The distorted Bi5+O6 octahedra formed one-dimensional chains via edge-sharing along the b-axis, with the chains held by Bi3+/Na+ split sites. The structural feature except for the split distribution of Bi3+/Na+ was classified as a Na2MnCl4-type structure. DFT calculations based on a model discounting the split distribution of Bi3+/Na+ indicated that Bi 6s and O 2p orbitals form sp hybridization at the conduction band. This new mixed valence bismuth oxide exhibited photocatalytic activity for phenol degradation under visible light irradiation. In addition to Na3Bi3O8, the hydrothermal reaction using NaBiO3 center dot nH(2)O in NaOH solution yielded micrometer-sized single crystals of an ilmenite-type NaBiO3 and two polymorphs of bismuth oxides with monodinic (alpha-Bi2O3) and orthorhombic (epsilon-Bi2O3) structures, depending on the reaction temperature and NaOH concentration.