The wetting of MgO and alpha-Al2O3 polycrystalline plates by molten Mg in purified argon was studied between 973 and 1273 K using an improved sessile drop method. The MgO/Mg system is basically non-reactive while Al2O3/Mg is reactive. The common features in the non-reactive and reactive systems are the high tendency for oxidation and the high evaporation rate of Mg. It is shown that wetting kinetics in both systems is governed by the evaporation and pinning phenomenon leading to contact angle versus time curves passing through a minimum: the apparent contact angle first decreases and then increases. The intrinsic contact angle should be estimated using the initial value (advancing contact angle) and the maximum value just before the disappearance of the droplet caused by Mg evaporation (receding contact angle).