Abstract A method for determining the concentration of segregated regions in molten binary alloys is given. The melt consists of a matrix containing both kinds of atoms in statistical distribution and, furthermore, two kinds of inhomogeneities, one kind containing only Cu and the other kind only Bi atoms. The method is applied to Bi-Cu melts. It is found that nearly all of the inhomogeneities consist of Cu atoms only. The volume concentration of these inhomogeneities within the melt depends on the initial composition of the melt and varies between 10 and 25 volume percent. From the temperature dependency of the coherent scattering cross section, it follows that the diameter of inhomogeneities remains constant during an increase in temperature of 300°C. The concentration of inhomogeneities, however, is reduced to SoOA of the original value. The determination of the derivative of thermodynamic activity with respect to the concentration obtained from scattering experiments shows qualitative agreement with the corresponding values determined directly by thermodynamic measurement. The mean lifetime of segregated zones can be estimated to be on the order of approximately 10−11 sec. Other physical data which were determined with Bi-Cu melts are discussed with regard to the strucrure of these melts.