Calcium selenate subhydrate, CaSeO(4)·0.625H(2)O, was prepared by hydrothermal conversion of CaSeO(4)·2H(2)O at 463 K. From the single crystals obtained in the shape of hexagonal needles, 50-300 µm in length, the crystal structure could be solved in a trigonal unit cell with space group P3(2)21. The cell was confirmed and refined by high-resolution synchrotron powder diffraction. The subhydrate was characterized by thermal analysis and Raman spectroscopy.