We present in this article a theoretical approach describing the viscosity increase in an overmelt liquid as the result of an augmentation of covalent bonds in the glass network being created, by means of an agglomeration of well-defined structural entities. Thus, one can study the characteristic temperature of this agglomeration process, identified as the glass-transition temperature, as a function of the modifier rate in a binary system. Result analysis and comparison with experiment points out that connectedness (and the subjacent entropy of the glass network) control mainly the variation of the glass-transition temperature with the modifier rate.