A small-angle X-ray diffraction study of the supercritical region of the B 2O 3PbOAl 2O 3 immiscibility surface is presented. This is the first time that diffraction methods have been applied to the study of critical opalescence in a system of oxides at high temperatures. Debye's theory is shown to be applicable and, from the spectra correlation lengths and the range of molecular interactions l is calculated. The topography of the supercritical region is determined and the relationship between l and the main interatomic distances in the melt discussed. The spectra of supercritical fluctuations in the liquid are compared to the spectra of vitreous specimens obtained by quench. This permits the quality of the quench to be assessed and an accurate determination made of the initial condition for the study of spinodal decomposition in the miscibility gap.