To the main advantages of magnesium alloys belongs their low density, and just because of such property the alloys are used in aviation and rocket structures, and in all other applications, where mass of products have significant importance for conditions of their operation. To additional advantages of the magnesium alloys belongs good corrosion resistance, par with or even surpassing aluminum alloys. Magnesium is the lightest of all the engineering metals, having a density of 1.74 g/cm3 . It is 35% lighter than aluminum (2.7 g/cm3 ) and over four times lighter than steel (7.86 g/cm3 ). The Mg-Li alloys belong to a light-weight metallic structural materials having mass density of 1.35-1.65 g/cm3 , what means they are two times lighter than aluminum alloys. Such value of mass density means that density of these alloys is comparable with density of plastics used as structural materials, and therefore Mg–Li alloys belong to the lightest of all metal alloys. In the present paper are discussed melting and crystallization processes of ultra-light weight MgLi12,5 alloys recorded with use of ATND methods. Investigated magnesium alloy was produced in Krakow Foundry Research Institute on experimental stand to melting and casting of ultra-light weight alloys. Obtained test results in form of recorded curves from ATND methods have enabled determination of characteristic temperatures of phase transitions of the investigated alloy.