Wettability between the liquid matrix and the reinforcement is important for the processing of metal matrix composites; the contact angle can be measured with the sessile drop method. However, for the wetting angle of liquid Al-Mg alloys on ceramics, it is difficult to obtain an accurate measurement because aluminum is easily oxidized and the rate of evaporation of Mg is high. Here, an oxide-stripping device is used to provide an aluminum alloy droplet with minimal oxide on the surface when the droplet makes initial contact with the substrate. Results are reported for contact angle (theta) versus time for Al-2.1, 3.9, 9.1, and 14.2 wt% Mg alloys with sintered silicon carbide substrates. The samples were quenched from the test temperature. Magnesium plays a key role in the adhesion, or otherwise, of the droplet to the substrate. Magnesium is probably migrating to the surface of the droplet and evaporating or forming a "crusty" layer on the surface over time. When viewed from above, "halo" regions, initially showing interference colors, develop on the substrate around the base of the droplets. These are probably associated with Mg (and Al) diffusing from the droplet across the surface of the substrate.