In-depth studies of property-composition relationship in chalcogenide glasses have revealed some interesting structural features. For example, specific discoveries in the Ge-Se-Sb and similar systems were of (i) meta-stable phase separation for glasses with a high Se or S content ( >70 at.% ), and (ii) abnormal behavior of some structure-sensitive properties ( e.g. optical band-gap energy ) for glasses with Se or S in the range of 60–70 at.%. In carefully studying the Ge-Se system, the liquidus curve ( reported by H. Ipser et al, 1982 ).was reexamined; this was found to have an S-shape at the phase-separation region. Another interesting finding was the dependence of the band-gap energy on composition in (1−x)GeSe2 xSb2Se3 glasses; this was not a linear relation but a curve with a maximum band-gap energy at 9 at.% Sb.