Bulk chalcogenide glasses are usually prepared by equilibrium melt cooling. An equimolar stoichiometric (As2Se3)(50)(Sb2Se3)(50) melt equilibrated at 850 degrees C was used and either quenched or slowly cooled. The basic thermoanalytical behaviour was studied and compared with the equilibrium phase diagram. Markedly different results were obtained. In the former case, the frozen state was completely glassy. On the base of DSC and Raman spectroscopy, the frozen melt structural units are close to the structural units in Sb2Se3, As3Se2 and glassy Se. In the latter case, one can conclude that the frozen state is mixture of both crystalline Sb2Se3 and the glass (instead of crystalline phase) close to the eutectic composition, i.e. As35.2Sb4.8Se60.0. It means that at 850 degrees C the basic structural units of As2Se3 (trigonal pyramids AsSe3/2) are no longer stable and decompose into As3Se2 and selenium. The cooling rate decides whether those products will be frozen in the structure (former case) or recombination of decomposition products take place (latter case). (C) 2011 Elsevier B.V. All rights reserved.