Quantitative criteria are discussed to determine whether the laws of diluted solutions are suitable for molten metals. To this end, the activity coefficient logarithms with a deviation of ±0.05 can be used. This deviation is specified by the International Union of Pure and Applied Chemistry as a permissible discrepancy between stability constants for metal complexes and indicated for reference. Data on electrochemical separation of cerium and yttrium are reported in the molten Kl-NaCl-LnCl3 system.