  We give in this letter a set of general rules which allow the prediction of
the value of the glass transition temperature $T_g$ in network glasses.
Starting from the Gibbs-Di Marzio law which gives a very general relationship
between this temperature and the average coordination number of a system, we
explain how to compute from the valencies of the atoms of the glass, the
parameter $\beta$ used in this law. We check the validity of the obtained
expressiosn and show that it is possible to predict the glass transition
temperature for any composition in multicomponent chalcogenide glasses. The
possibility of existence of a demixed structure is also discussed.
