Bismuth niobates were obtained by hydrothermal synthesis using bismuth (III) nitrate pentahydrate [Bi (NO3)(3))center dot 5H(2)O] and a Lindqvist potassium hexaniobate [K(8.x)H(x)Nb(6)O(19)(center dot)nH(2)O] as soluble precursors. Pure crystalline Bi5Nb3O15, beta-BiNbO4 and alpha-BiNbO4 were obtained as pure phases through co-precipitation at pH = 8, followed by hydrothermal treatment and calcination at 600, 700 or 800 degrees C, respectively, which are milder temperatures than those used in solid state, citrate sol-gel and ball milling methods. Band gap energies of alpha-BiNbO4 and beta-BiNbO4 were estimated by UV-vis diffuse reflectance spectroscopy (DRS) as 3.2-3.4 eV, which are higher than experimental values reported in the literature. The materials were evaluated in the methyl orange azo-dye photo-discoloration under UV irradiation and complete bleach was attained. The triclinic phase presented a better efficiency as a photocatalyst than the orthorhombic phase. Unexpectedly, the mixture of both phases resulted in the highest photocatalytic efficiency, similar to that found for Degussa P25 commercial catalyst.