Viscosity changes of RO-SiO₂-Fe_xO (R = Ca, Sr, Ba) melts due to changes in the oxidation states of the iron ions by systematically varying the oxygen partial pressure were measured at 1773 K. The initial compositions of the samples were 30RO-60SiO₂-10Fe₂O₃(mol%), and the ratio of Fe³⁺ to Fe²⁺ in the RO-SiO₂-Fe_xO melts increased with increasing oxygen partial pressure in all samples. Meanwhile, the viscosity of all the RO-SiO₂-Fe_xO melts decreased with increasing Fe²⁺ to total-Fe ratio (Fe²⁺/t-Fe). The data indicate that the increase in the amount of Fe²⁺ ions, which behave as network modifiers, would result in depolymerization of the silicate anions. In addition, the viscosity of the melts increased in the order of alkaline-earth cationic radius (Ba > Sr > Ca) when the Fe²⁺/t-Fe values of the melts were comparable. This would be due to the change in the coordination structure of Fe³⁺ in the melts.