[en] The melting points of some lead and zinc salts, that can be used as solvents for ferrites in systems: PbMoO₄-MgFe₂O₄, Zn₂V₂O₇-NiFe₂O₄, Pb₃(VO₄)₂-MgFe₂O₄, have been calculated in accordance with the hypotetical mechanism of the solvent dissociation. The accuracy of cryoscopic calculations based on melting point curves is evaluated. Cryoscopic calculations permit to determin the solvent activity with the accuracy of +-0.3% and the heat of its fusion, with the accuracy of +-3%. The comparison of the calculated and experimental values of the entropy of melting, as well as calculated and experimental values of the cryoscopic constant elucidates the mechanism of dissociation of both the dissolved compound and the solvent