The crystal structure, thermoanalytical properties, and infrared absorption spectra of (\mathrm{Co}\left(\mathrm{HSeO}{3}\right){2} \cdot 2 \mathrm{H}{2} \mathrm{O}) are given. The crystals of the substance are monoclinic, with space group (P 21 / n, a=) (7.1357(6) \AA, b=6.8759(6) \AA, c=8.0325(8) \AA, \beta=113.19(1)^{\circ}), (V=362.27(6) \AA^{3}, Z=2), and (R=0.020) for 596 observed reflections. The cobalt atom is located in the center of the coordination octahedron consisting of the oxygen atoms from four hydrogen selenite anions and two water molecules. The structure is linked together by means of a system of hydrogen bonds including those among the (\mathrm{HSeO}{3}^{-})anions and between the (\mathrm{HSeO}{3}^{-})anions and water molecules. The thermal decomposition of (\mathrm{Co}\left(\mathrm{HSeO}{3}\right){2} \cdot 2 \mathrm{H}{2} \mathrm{O}) begins at (400 \mathrm{~K}) by gradual liberation of water molecules, leading finally to the formation of cobaltous diselenite (up to (580 \mathrm{~K}) ). A further increase in the temperature causes stepwise liberation of selenium dioxide, with formation of cobaltous selenite (up to 760 K) and then cobalt(II) oxide (up to (950 \mathrm{~K}) ). No phase transition connected with a change in the proton arrangement was found by vibrational spectroscopy at temperatures down to (77 \mathrm{~K}). The existence of ferroelectric properties of the (\mathrm{Co}\left(\mathrm{HSeO}{3}\right){2} \cdot 2 \mathrm{H}_{2} \mathrm{O}) crystals can be excluded at laboratory temperature, owing to centrosymmetry of the space group. (O 1994) Academic Press, Inc.