The results obtained by many authors on the GeSeIn vitreous system, in particular, those concerning the variations of the glass transition temperature, Tg, with the average coordination number, Z, are investigated from a new perspective. From the assumption that In atoms participate in the formation of In2Se3microclusters, we show that the variations of Tg with Z of the alloy are consistent with Sreeram's relationship and with Tanaka's structural transition model. We conclude that In is threefold coordinated with Se atoms to form In2Se3 microclusters and that these microclusters do not contribute to the variations of Tg with the alloy composition.