The melting of coarse- and fine-grained Al–Mg alloys was studied by the differential scanning calorimetry (DSC). The transition from incomplete to complete wetting of grain boundaries (GBs) takes place in binary Al–Mg alloys. The heat effect of GB wetting manifests itself in the asymmetric shape of the DSC melting curve. By decreasing grain size (and increasing specific area of GBs) the GB “shoulder” in DSC curve becomes more pronounced. Basing on DSC data, the position of GB solidus (or premelting) line for Al–Mg alloys has been estimated. Contrary to the Al–Zn alloys, the difference between bulk and GB solidus does not exceed 2 °C.