The process of unbalanced crystallization of industrial aluminium alloys AD31 (6063) and AD31 (6061) of the Al — Mg – Si system, was studied by the thermal analysis method with the following numerical time differentiation of culling curves. There were established the temperatures of liquidus and solidus of the mentioned alloys, along with the temperatures of ternary phases (Al – Fe – Si), which were formed in the alloys by exothermal peritectic reaction between liquidus and solidus. The method of numerical X-ray microanalysis was used to define the local chemical composition of metastable phases (Al – Fe – Si) and as-cast aluminium alloy matrix. The small content of impurity elements was found out in the metastable phases (Al – Fe – Si). During the consolidation of alloy AD31, the acicular shaped metastable phase Al 17Fe 2Si 3 was released with the temperature by 594 оС. The dispersed particles of this metastable phase were identified in the microstructure of alloy AD31, which contains 0.12% of Fe. The alloy AD31 reveals the metastable phase Al 8FeSi, which was released from the melt with the temperature by 619 оС, and has a shape of Chinese hieroglyphs. In comparison with the alloy AD31, the microstructure of alloy AD33 contains much more Fe (0,57%). Because of the unbalanced crystallization conditions, the chemical composition of the metastable phases (Al – Fe – Si) significantly differs from the composition of certain equilibrium α- and β-phases (Al – Fe – Si), which is peculiar to the alloys AD31 and AD33. The dispersed eutectic release of Mg 2Si phase is situated on the boundaries of dendrite cells and was identified in the microstructure of casted alloys. However, the volume fraction of Mg 2Si phase is small. Magnesium and silicon are situated mainly in oversaturated solid aluminium-based solution, as a result of the unbalanced alloy crystallization conditions.