The wetting behavior of solid Al₂O₃ with molten CaO-Al₂O₃-SiO₂ was investigated at 1 873 K using the sessile drop method. A new model was developed to represent the time dependence of the contact angle, i.e., the spreading behavior of a liquid drop on a solid substrate. The model takes into consideration chemical interactions which continually take place at the interface between the solid Al₂O₃ and molten CaO-Al₂O₃-SiO₂. By applying the model to the experimental results of the present study the equilibrium contact angle between the liquid slag and solid alumina was determined for a number of different slag compositions, and an iso-contact angle diagram was constructed. The equilibrium contact angle was greatly affected by the slag composition, and it was found that the interfacial tension was the major factor governing the equilibrium contact angle. In the region of low SiO₂ content, the slag with higher CaO content exhibits a smaller contact angle, i.e., better wettability with alumina. For slag with a given CaO/SiO₂ ratio, an increase in Al₂O₃ results in a corresponding increase in the contact angle, i.e., decrease in wettability. For a given CaO/Al₂O₃ ratio, the variation of the contact angle with SiO₂ content shows a minimum. The contact angle decreases by increasing the surface roughness of the alumina substrate.