In order to understand the effect of slag composition on foaming in the smelting reduction process, slag foaming was quantitatively studied for CaO–SiO2–FeO slags in the temperature ranging 1250–1400 °C. It was found that slag foaming could be characterized by a foaming index Σ which is equal to the retention or travelling time of the gas in the slag and by the foam life. The effects of P2O5, S, MgO and CaF2 on foaming were studied. As expected slag foaming increased with increasing viscosity and decreasing surface tension. The results were extrapolated to bath smelting process to predict the foam height. Slag foaming heights as high as 3–5 meters are predicted for a typical operation.