The wettability of liquid slag from the CaO-SiO2-MgO-Al2O3 system on a solid Al2O3 substrate at 1773 K has been investigated using the sessile drop experimental technique. The liquid slag compositions were chosen to represent blast furnace hearth slags and test the effects of changing the Al2O3 (5.2-18.9 pct) and V ratio (0.41-1.17) of the slag on its wettability on solid Al2O3. All slag compositions tested were wetting with respect to solid Al2O3, and the contact angle (θ) changed with time, rapidly decreasing in ~10–15 seconds from an initial value between 40 and 80 deg to a plateau value of ~18 to 19 deg at extended times. There was no obvious effect of changing the initial Al2O3 content of slag on the wetting behavior, but increasing the initial V ratio of the slag lowered θ. Similarly, there was no obvious effect of changing the initial Al2O3 content of the slag on spreading rate but the slags with the higher V ratio had a faster spreading rate. The apparent of adhesion (W) of the slags on the solid Al2O3 was also evaluated and found to increase for increasing Al2O3 content of the slag and increasing the V ratio of the slag. This higher W indicates a stronger bond between the slag and solid Al2O3 substrate. Using electro-optical techniques to characterize the slag-substrate interface, it was established that the slag had penetrated and reacted with the solid Al2O3 and formed a layer. As a result of this penetration/reaction, new phases formed. The specific phases that formed were dependent on the initial slag composition. In the changing initial Al2O3 slags at a relatively constant V ratio of 1.1 to 1.2, calcium aluminates, CA2 and CA6, were found. At the lower V ratios (0.41 and 0.60) but constant Al2O3, compositions consistent with spinel, feldspar and melilite were found.