[en] The thermodynamic analysis of the behaviour of boiling water reactor coolant on the basis of solubility in Fe₃O₄-H₂O-O₂ system is performed for the purpose of establishing the iron existence forms in non-sedimentated suspended corrosion product particles as well as iron concentration of corrosion origin in power plants. It is shown that the iron solubility in the considered system with temperature variation occurs through the maximum at 423 K. Below this temperature the crystal Fe(OH)₃ is responsible for its value, at higher temperatures - magnetite. The growth of equilibrium oxygen concentration from 0.1 to 1000 μg/kg H₂O only slightly increases the magnetite solubility