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Bulk glasses Ge x Sb 0:05 S 0:95Àx and Ge x Sb 0:1 S 0:9Àx were prepared. Both the optical gap E 03 and the compactness C dependence against the mean coordination number hCNi indicates threshold-like behavior around Tanaka's threshold. Using simple covalent bond statistics the far infrared spectra are discussed in relation to possible bonding arrangements in the studied glasses. The analysis reveals (i) preference of Ge±S bond formation at the expense of Sb±S bond formation in chalcogen poor glasses, and (ii) the threshold-like behavior in E 03 and C dependences against hCNi can be attributed to the chemical threshold, that is to such chemical composition and bonding arrangement where the system reached the most stable state with respect to formation energy.

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