In the present work, the early stage of clustering and the subsequent precipitation of Si atoms in Al + 1.37, Al + 2.83 and Al + 6.81 at.% Si alloys have been investigated by means of electrical resistivity and microhardness measurements. The results showed that: (i) the interaction between the vacancy- type dislocations and the partially precipitated Si atoms is the predominant process during the initial stage (25-150 °C); (ii) as the large particles of Si precipitates are formed, they become no longer coherent with the Al matrix; (iii) the microhardness of the as-quenched specimens was found to increase linearly with increasing Si concentration in the alloy.