Slag foaming measurements in terms of the foaming index (∑) were conducted on bath smelting-type slags (CaO-SiO2-FeO, CaO-SiO2-MgO-Al2O3-FeO) at 1773 K. It was found that the slag foam stability decreases with increasing FeO (FeO > 2 pct) content and basicity. For the slag system (CaO-SiO2-FeO), no stable foam was observed at very low FeO content (<2 pct). As pct FeO increases, the slag foaming index goes through a maximum and then decreases; a similar phenomenon was observed for CaO-SiO2-NiO slags with respect to the NiO content. The foaming index determined from the normal small-scale experiments (3.8-cm ID diameter) were confirmed on a larger scale (9.2-cm ID diameter), indicating that the foaming index is independent of container size. Measurements were also made for the actual compositions for bath smelting slags. For these slags, the foaming index is higher than those of simple CaO-SiO2-FeO slags, because MgO and Al2O3 may increase their viscosities. The foam index is believed to be a function of the physical properties of the slag. Consequently, a dimensional analysis was performed, and a correlation was developed relating the foaming index to the viscosity, surface tension, and density of the slag. An estimation of slag foaming in actual pilot plant trials was also made from the results of the present study. Good agreement was observed between the predicted and observed foam heights and indicated coke in the slag can reduce the foam height by more than 50 pct.