BiTa1-xNbxO4 (0 less than or equal to x less than or equal to 1) solid photocatalysts were prepared by solid-state reaction and characterized by powder X-ray diffraction and Rietveld structure refinement. The structure of BiTa1-xNbxO4 (x = 0.0 and 0.5) is triclinic. However, the structures of x = 0.2, 0.8 and 1.0 are orthorhombic. The H-2 evolution was obtained from an aqueous CH3OH/H2O solution and from pure H2O with BiTa1-xNbxO4 (0 less than or equal to x less than or equal to 1) under UV irradiation. The orthorhombic photocatalysts exhibit much higher activity than that of triclinic photocatalysts. The orthorhombic photocatalyst at x = 0.2 showed the highest activity. An UV-vis diffuse reflectance spectroscopy measurement revealed that the band gap of orthorhombic photocatalysts is more narrow than that of triclinic photocatalysts.