The reactive wetting behavior of molten CaO-SiO2-Al2O3-MgO slag drops on a coke substrate was investigated using the sessile drop method at 1500 A degrees C in flowing Ar. The wetting behavior of slags in blast furnaces is known to affect the energy efficiency of ironmaking, but has not yet been sufficiently characterized. In this study, the ratio of MgO to CaO was varied from 0.01 to 0.56, while the concentrations of SiO2 and Al2O3 were fixed at 34 and 17 wt%, respectively. The contact angle between slag and coke remained constant for a certain period of time, and then suddenly decreased remarkably. The time until this dramatic contact angle change increased with increasing the ratio of MgO to CaO. Chemical analysis revealed that the reduction of SiO2 in the slag improved the wettability by forming SiC on the coke substrate, while MgO in the slag was preferentially reduced and prevented the reduction of SiO2. Consequently, with increasing MgO content, the wettability of the molten slag on the coke substrate decreased.