Slag viscosity as a function of temperature and composition is a very important factor in determining the operating temperature, blended coals as well as fluxing agents for slagging gasification. A number of models for predicting the viscosity of fuel slags have been developed, however, most of them are only valid in a limited range of temperatures and compositions. This study aims at developing a new viscosity model for fully liquid slag systems in the Newtonian range, based on the thermodynamic modified associate species model. The viscosity model is a structurally-based model, which gives a reliable prediction over the whole range of compositions and a broad range of temperatures. The focus of the present paper is to collect and model the experimental data for pure oxides SiO2, Al2O3, CaO, MgO, Na2O, K2O and binary systems SiO2-Al2O3, SiO2-CaO, SiO2-MgO, SiO2-Na2O, SiO2-K2O, Al2O3-CaO, Al2O3-MgO, Al2O3-Na2O, Al2O3-K2O, which is the first step to develop a new viscosity model for the system SiO2-Al2O3-CaO-MgO-Na2O-K2O. A good agreement between experimental data and calculated data has been achieved using only one set of model parameters, which have a clear physico-chemical meaning. Moreover, the extrapolated viscosities to the regions where no experimental data are available in literature are reasonable. (C) 2015 Elsevier B.V. All rights reserved.