The composition dependence of viscosity in binary silicate liquids is approximated by a linear equation of the form ln h = Si Xi Di, where h is the Newtonian viscosity coefficient, Xi is the mole fraction of oxide component i, and Di is an empirical constant associated component i over a restricted range of SiO 2 concentration. This relation has been extended to multicomponent silicate liquid systems, and a preferred set of viscosity measurements (2440 data points) has been used to evaluate Di constants at 50 degrees C intervals over the temperature range 1200 degrees to 1800 degrees C. Di values for the components SiO 2 , TiO 2 , FeO, MnO, MgO, CaO, SrO, BaO, Li 2 O, Na 2 O, K 2 O. KAlO 2 , NaAlO 2 , BaAl 2 O 4 , CaAl 2 O 4 , MgAl 2 O 4 , and MnAl 2 O 4 are given for mole fraction SiO 2 intervals 0.35 to 0.45, 0.45 to 0.55, 0.55 to 0.65, 0.65 to 0.75, and 0.75 to 0.81. The model may be used to calculate the viscosities of many natural magmatic liquids. A comparison of calculated viscosities with all the published data for molten rock and mineral systems shows a very satisfactory agreement over a range of viscosities from approximately 1 to 106 poises.