Grain boundary wetting transitions were previously observed in the Al-Mg system. The melting of as-cast Al-5 wt% Mg and Al-10 wt% Mg alloys was studied by the differential scanning calorimetry. The asymmetric shape of the melting curve permitted the observation of the thermal effect of grain boundary wetting. The difference in the shape of the melting curve for the two studied alloys is explained by the different temperature dependence of the fraction of completely wetted grain boundaries.