Liquid-liquid structural transitions have been investigated by means of the emanation method in Fe–S, Co–S, Ni–S melts near the eutectic point. Both polythermal and isothermal conditions were used in the experiments. The superheating values were 0–300 ° above the melting temperature. In order to calculate the diffusion coefficient of220Rn in melts a model connecting the emanation level with the diffusion coefficient of radon was employed. Using the diffusion coefficient vs. viscosity relationship /the Stokes-Einstein relation/ the viscosity of melts as a function of the composition and temperature was obtained. Cluster-formation in sulphide melts was estimated theoretically, the particle number in a cluster as a function of the composition was determined, and a correlation was established between viscosity and particle number in clusters vs. composition.