The solubility of MnSeO3-SeO2-H2O system was studied in the temperature region 25–300°C. The compounds of the three-component system were identified by the
Schreinemaker’s method. The phase diagram of manganese(II) selenites was drawn and the crystallization fields for the different
phases were determined. Depending on the conditions for hydrothermal synthesis, MnSeO3·H2O, MnSeO3·3/4H2O, MnSeO3·l/3H2O and MnSe2O5 were obtained. The different phases were proven and characterized by chemical, powder X-ray diffraction and thermal analyses,
as well as IR spectroscopy. The kinetics of dehydration and decomposition of MnSeO3·H2O was studied under non-isothermal heating. Based on 4 calculation procedures and 27 kinetic equations, the values of activation
energy and pre-exponential factor in Arrhenius equation were calculated for both processes.