Understanding mechanisms of selenium's behavior in the near-surface conditions is an actual problem of the modern mineralogy and geochemistry, and it is very important for solving some environmental challenges. The aim of the present work is to define the Gibbs energy of formation (Δ f G 0 ) for synthetic analogues of three minerals: cobaltomenite (CoSeO 3 2H 2 O), alpheldite (NiSeO 3 2H 2 O) and chalcomenite (CuSeO 3 2H 2 O) on the base of their solubility in water. Solubility was determined by the isothermal saturation method in ampoules at temperature 25 °C. Calculations of ions' activities in the solution in equilibrium with solid phases were spent by means of the software package Geochemist's Workbench (GMB 9.0). It has been revealed that dissolution of cobaltomenite, alpheldite and chalcomenite is the incongruent one and accompanied by formation of Co, Ni and hydroxides. Detected Δ f G 0 values are: CoSeO 3 2H 2 O = -937.5 kJ/mol, NiSeO 3 2H 2 O = -931.3 kJ/mol, and Cu-SeO 3 2H 2 O = -836.3 kJ/mol. These data are well corresponding to the Δ f G 0 values obtained by calorimetric investigations of thermodynamic characteristics of synthetic analogs of cobaltomenite, ahlfeldite and chalcomenite.