Abstract: The dissolved rare earth content in iron was determined by means of electrolysis of the sample in the organic electrolyte at low temPera-ture and radioassay technique. The oxygen activity in the molten iron was measured by solid electrolyte sensors made of ZrO₂ (MgO) tube. The dissolved sulphur content was determined by aid of H₂S evolution method. The relationship between the Y-O equilibrium constant in molten iron and temPerature at 1560-1600℃ and the Y-S equilibrium constant in molten iron at 1600℃ were determined. The interaction coefficients of eOY, eSY and other thermodynamic data were also determin-ed.