In the present work the intermetallic phases in an Al corner of the Al-Fe- (Mn)-Si diagram were analysed in the area of the co-existence of the ternary α-AlFe (Mn) Si and quaternary α-Al(FeMn) Si compounds. The microstructure of the alloys, chemical composition of the phase constituents and thermal effects during continuous slow cooling were examined using microscopic observation, X-ray microanalysis and differential scanning calorimeter. The phase components present in the alloy AlSi2.5(Fe+Mn)12 (where Mn = 0.03, 0.25, 1.8, 3.0% mass.) were identified on the base of their chemical composition. The successive progress of the peritectic trans-formation L+Al3Fe (Mn, Si) - α-Al. +α-Al (FeMn) Si was shown as the Mn content in an alloy increased until the primary precipitation process of the quaternary α-Al (FeMn) Si phase started. The chemical composition of the phase components was estimated during the progress of the phase transformation. The content of Si in the primary Al3Fe phase was evaluated to be in the range 0.9-2.0% mass. Simultaneously, the Mn presence (0.5-8.5% mass. increasing with an increase in the Mn content in the alloy) was stated in this primary phase. A limit of the Mn concentration in the alloy promoting the process of the primary precipitation of the α-Al (FeMn) Si phase was established as 2.5% mass. This Mn effect was enhanced by 1.3% Cr addition into quaternary alloy. The temperature and the sequence of the phase transformations during slow cooling (5K/min) of the examined alloys were identified. The temperature range of the primary precipitation of the binary, ternary and quaternary intermetallic phases, influenced by the Mn content in the alloy, was estimated.