The effect of the oxidation state of iron on the phase separation of xNa(2)O . (100 - x)SiO2 glasses, x = 18.56 and 13, containing 0.5 mol% Fe2O3 was studied. The oxidation state of iron in the glasses was varied by changing the melting conditions, such as melting temperature and melting atmosphere. The oxidation states of the iron ion were determined using colorimetric and UV-VIS-NIR spectrophotometric methods, and a comparison was made between the results obtained using these two methods. Immiscibility temperatures of the glasses were determined using opalescence and clearing methods. The immiscibility temperature of the sodium silicate binary glasses decreased similar to 25 degreesC with the addition of 0.5 mol % Fe2O3. The immiscibility temperature of the doped glasses increased slightly with increased concentration of Fe2+ ion in the glass. The prediction of immiscibility tendency on the addition of a minor amount of third component was made using models proposed by Tomozawa and Obara and Nakagawa and Izumitani. The Tomozawa and Obara model showed good agreement with measured immiscibility values.