The stable water-soluble tantalum complex with lactic acid (ammonium tetralactatodiperoxo-mu-oxo-ditantalate(V)), (NH4)(4)[Ta-2(C3H4O3)(4)(O-2)(2)O]center dot 3H2O, was prepared in the crystalline form. According to the single-crystal X-ray diffraction data, this compound forms a monoclinic cell with a = 13.85(2) angstrom, b = 9.06(1) angstrom, c = 12.32(2) angstrom, and beta = 116.30 degrees, space group C2 (No. 2), and has Z = 2 molecules per unit cell. The solid-state C-13 NMR data and low flack parameter are consistent with the determined structure. Appearance of the same vibration modes in Raman and IR spectra supports the choice of the space group without inversion symmetry. The solution of the tantalum complex was successfully applied for the synthesis of two photocatalytic materials, NaTaO3 and Sr2Ta2O7.