Experiments based on exposure of MgO to slags under forced convection flow conditions allowed the identification of different degradation mechanisms and the assessment of the role of Al2O3 in the degradation process. Slag with no alumina present resulted in direct dissolution. Samples immersed in alumina containing slag underwent indirect dissolution, with a spinel forming at the MgO-slag interface. At 1530 °C, the spinel was not effective in reducing the corrosion rate, as the scattered spinel grains were easily removed from the MgO surface. At 1500 °C, the loss of MgO was reduced due to the formation of a more cohesive spinel layer. Mechanical erosion then appears to play a greater role. Strength of the bond between the spinel and underlying MgO needs to be considered in strategies to reduce degradation of MgO refractories.