The melting of coarse- and fine-grained Al-Zn-Mg alloys was studied by the differential scanning calorimetry. The asymmetric shape of the melting curve indicates the heat effect of the grain boundaries (GBs) wetting. The difference between melting heat of coarse- and fine-grained samples permitted to estimate the GB wetting heat effect per GB unit area. It is ~0.15 J/m² for the Al-5 wt% Zn-2 wt% Mg alloy and ~1 J/m² for the Al-10 wt% Zn-4 wt% Mg alloy.