Aluminum-Silicon alloys are sought in a large number of automotive and aerospace applications due to their low coefficient of thermal expansion and high wear resistance. The fine structure of Si precipitates is controlled by forced solid solution structure obtainable by rapidly solidification techniques [1, 2, 3]. Present study focuses on precipitation of silicon in Al88Si12 as a function of temperature by dilatometer analysis. Different structures out of equilibrium have been obtained after casting in sand, black-lead and steel mould and by melt spinning. The average value of the activation energy for the precipitation of Si in steel mould casting of eutectic composition was found to be 47 kJ/mol. Our dilatometer studies were complemented by metallographic microscopy and XRD measurements.