New measurements have been made on the ferric to ferrous ratio as well as the sulphide capacity for platinum group metals (PGM) melter-type slags. In South Africa, these slags are produced from the smelting of low-grade copper-nickel sulphide ores, Nell [Nell, J., 2004. Melting of platinum group metal concentrates in South Africa. The South African institute of Mining and Metallurgy 104 (7), 423-428]. The typical mass Compositions are 5-10% Al(2)O(3), 2-15% CaO, 5-30% FeO(x), 15-25% MgO and 40-60% SiO(2) with a molar basicity defined as (CaO + MgO)/SiO(2) of 0.6-1. The industrial furnaces operate at temperatures ranging from 1450 to 1600 degrees C under fairly reducing conditions (typically a pO(2) close to 10(-8) atm at 1500 degrees C). The gas-slag equilibrium was studied by subjecting a synthetic slag to controlled atmospheres in a vertical tube-furnace using Ar-CO-CO(2) (-SO(2)) gas mixtures. The ratio of ferric to ferrous was determined at 1450 degrees C for oxygen activities, defined as pCO(2)/pCO, ranging from 0.11 to 1.75 by analysing the quenched slags using the standard titration and XRF techniques. The measured Fe(3+)/Fe(2+) ratio increased from 0.029 to 0.110 with the increasing oxygen activity. Slight non-ideal iron redox behaviour was observed, as has been reported for low alumina and low iron-containing slags. The present results are in good agreement with the trends found in the literature for similar multicomponent slag systems (mostly iron bath smelting slags). Sulphide capacity was measured at partial pressures of oxygen and sulphur of approximately 10(-9) and 10(-3) atm respectively, with total-iron contents of 8.2 and 15.6 wt%, and temperature ranging from 1450 to 1525 degrees C. The present sulphide capacity data ranged from 10(-4.43) to 10(-3.71). The expected increase in sulphide capacity with increasing temperature was observed, and at a given temperature, the sulphide capacity increased with an increase in iron oxide content. (c) 2005 Elsevier Ltd. All rights reserved.