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The viscosities of two melts in the system Na2O-FeO-Fe2O3-SiO2 have been measured as a function of oxidation state. The experiments were conducted by concentric-cylinder viscometry, on melts equilibrated with gas mixtures in a vertical tube, gas-mixing furnace. Viscosity determinations were made during stepwise reduction and oxidation of the melts. 57Fe M\xf6ssbauer spectra were obtained on quenched melt samples recovered during the viscometry experiments. In addition, a series of loop fusion experiments were performed at calibrated \u0192O2 values in order to relate viscosity determinations directly to \u0192O2.\n\nThe viscosities of acmite and NS4F40 (Na-rich and Si-poor relative to acmite) melts decrease with reduction of Fe in the melts, as nonlinear functions of , yielding a region of viscosity invariance at moderate to low values of (<0.4). The 57Fe M\xf6ssbauer spectra of quenched melts as a function of indicate the presence of one (network-modifying) ferrous species and two ferric species with ferric iron acting dominantly as a network-former in oxidized melts and dominantly as a network-modifier in reduced melts.\n\nThe presence of two ferric iron species produces a minimum in the degree of polymerization of the melt at intermediate values of : the region of viscosity invariance corresponds to this minimum. If viscosity is positively correlated with polymerization for all values of then the viscosity of very reduced melts will increase with reduction, as the melt polymerizes.\n\nThe effect of oxidation state on viscosity is large and illustrates that ferric iron should be considered as a separate component in calculation schemes for estimating the viscosity of natural magmas.