Thermophysical and thermochemical calorimetric investigations were carried out on the synthetic analogue of mandarinoite. The low-temperature heat capacity of Fe2(SeO3)3∙5H2O(cr) was measured using adiabatic calorimetry between 5.3 and 324.8 K, and the third-law entropy was determined. Using these Cp,mo(T) data, the third law entropy at T = 298.15 K, Smo, is calculated as 520.1 ± 1.1 J∙K–1∙mol–1. Smoothed Cp,moT values between T →0 K and 320 K are presented, along with values for Smo and the functions [HmoT-Hmo0] and [ΦmoT-Φmo0]. The enthalpy of formation of Fe2(SeO3)3∙5H2O(cr) was determined by solution calorimetry with HF solution as the solvent, giving ΔfHmo(298 К, Fe2(SeO3)3∙5H2O, cr) = –3124.6 ± 5.3 kJ/mol. The standard Gibbs energy of formation for Fe2(SeO3)3∙5H2O(cr) at T = 298 K can be calculated on the basis on ΔfHmo(298 К) and ΔfSmo(298 К): ΔfGmo(298 К, Fe2(SeO3)3∙5H2O, cr) = ‒2600.8 ± 5.4 kJ/mol. The value of ΔfGm for Fe2(SeO3)3·5H2O(cr) was used to calculate the Eh–pH diagram of the Fe–Se–H2O system. This diagram has been constructed for the average contents of these elements in acidic waters of the oxidation zones of sulfide deposits. The behaviors of selenium and iron in the surface environment have been quantitatively explained by variations of the redox potential and the acidity-basicity of the mineral-forming medium. These parameters precisely determine the migration ability of selenium compounds and its precipitation in the form of solid phases.