The effect of Fe3+/Fe2+ redox equilibrium in ferrosilicate melts and ash slags on their viscosities is discussed. Viscosity experiments were carried out on two ash slags with low (Rochelle) and high (Illinois No. 6) iron content in CO2/CO (40/60) and air atmospheres as a function of temperature and as a function of time at a constant temperature. The lower values of viscosities measured in both ash slags under reducing conditions are assigned to the transformation of network-forming ferric iron to the network-modifying ferrous iron. Anomalies in viscosity are apparent in the reduced form of Illinois No. 6 ash slag during its oxidation in air at 1370 degrees C (slightly above the temperature of critical viscosity of the oxidized form of slag). First the viscosity significantly increases and then decreases to that of the oxidized form. The Fe3+/Sigma Fe ratio increases from 0.12 to 0.84. The conclusion is that changes of local configuration in the melt (coordination number and charge of iron ions and stability and size of crystalline phases such as hematite and quartz) determine the viscosity in Illinois No. 6 ash slag. Redox ratios of iron and structural positions of Fe3+ and Fe2+ ions in quenched slags were determined with Mossbauer spectroscopy.