The precipitation sequency of two casting alloys, namely the A356 and A357 AlSiMg, and the corresponding composites reinforced by 20% of Al2O3 have been studied by joint DSC and TEM in the underaged T6 state. Being Mg, combined with Si, the main hardening element of such alloys, it's actually interesting to monitor the phase transformations due to both the difference in Mg concentration in the monolithic alloys and to the possible Mg segregation in the composites. It has been verified that the precipitation sequency in the monolithic alloys occurs through the formation of beta", beta' and Si; a minor Si precipitation is observed in the alloy containing more Mg, due to the lower amount of free Si available. In the corresponding composites, the homogeneous nucleation of beta" is largely suppressed, and beta' precipitates are forming in limited quantity. A higher density of beta' is observed in the composite more rich in Mg, and Si particles are present in both the materials. The age hardening response of the materials is discussed in view of the suppression of pre-precipitate solute clustering processes.