This paper describes how the fragility of a liquid is linked to the ratio between the energy barrier (E eq ) for the equilibrium viscous behavior and that (E iso ) for the non-equilibrium iso-structural viscous behavior. Using the concept of iso-structural viscosity, two functions describing the variation of the configurational entropy (S c ) with temperature (T) are obtained from the Avramov-Milchev (AM) and the Vogel-Fulcher-Tammann (VFT) viscosity equations, respectively. The two S c (T) functions exhibit different relations to the liquid fragility. The AM S c (T) function is a power function with the exponent of F−1, where F is the AM fragility index. In this case, S c vanishes at T=0K. For the VFT function, S c vanishes as T is lowered to a finite temperature T 0 , whereas it reaches the maximum value S c,max at infinitively high T. S c,max is proportional to the VFT fragility index. Thus, the VFT equation is not only a dynamical, but also a thermodynamic model. It is proved that for oxide liquids, the VFT equation describes viscosity data better than the AM equation, provided the pre-exponential factor η 0 is fixed to a generally accepted value, e.g., 10 −3.5 Pas.