The viscosity of silicate melts is a parameter of extreme importance, not only in the Earth sciences, but also in a number of applied disciplines such as glass, and steel making. Empirical predictive models of melt viscosity proposed in the geological literature (e.g. Bottinga and Weill 1972), have been shown to be reliable at temperatures well above the glass transition, but these models do not take good account of non-Arrhenain behaviour, and extrapolation to temperatures close to the glass transition can lead to significant errors in estimated viscosity. In contrast to these empirical approaches, various attempts have made to quantitatively link viscosity to some structural property of the melt. Of these, the AdamGibbs theory (Adam and Gibbs, 1965) which considers viscosity to be related to the configurational entropy of the melt, has been shown to be particularly successful in accounting for nonArrhenian variations of viscosity of molten silicates with temperature (Richet, 1984; Bottinga et al., 1995). Despite this fact, the Adam-Gibbs (A-G) theory has not yet been used in a predictive sense, due to highly non-linear variations of the A-G parameters (Ae, Be, temperature independent constants, Sr the configurational entropy at the glass transition) as a function of composition, even along simple pseudo-binary joins, (e.g. CaSiO3MgSiO3, Neuville and Richet, 1991). Compositional dependence, and parameterisation of the A-G parameters As discussed by Toplis (1998), consideration of the A-G theory shows that non-linearities in the A-G parameters are to be expected due to the presence of terms involving the configurational entropy. However, the ratio of parameters Be and Sc(Tg) should show simpler variations as a function of composition, because terms involving the configurational entropy cancel out. The expression for the ratio BJSc(Tg ) is: