The non-equilibrium variant of the ENT method under continuous temperature decrease at a rate of 5-7 degrees C per minute was used for the first time to study the thermodynamic properties of Mg solid solutions in Al. This allowed to establish a valid relation between the abrupt change of the potential of Al-Mg alloy with Mg content 20-30 mol. % at 370-380 degrees C and the disintegration of the solid solution formed at 450 degrees C.