The decay kinetics and nonequilibrium state parameters of supersaturated substitutional solid solutions Al1-xSix (x<20 at. % Si) obtained by high-pressure synthesis have been studied comprehensively by resistivity relaxation and differential scanning calorimetry. It was shown that the decomposition process can be divided into two main stages. The first one consists of nucleation, growth, and coarsening processes usually observed during first-order phase transformations in metals and alloys. On the second stage the experimental data for isothermal relaxation and isochronal heat release can be explained assuming a nontraditional exp(-t/tau)(alpha) kinetics with exponent alpha approximate to 1/3. The drastic enhancement of the heat-release contribution at the latest stage of the phase separation is observed for x approximate to 6-8 at. % Si. Further increase of Si concentration in the face-centered cubic (fcc) lattice of an Al-based solid solution leads to the formation of fee Si clusters and initiates the splitting of the first stage of the decomposition process into three different branches. For any initial Si content, the second stage of the Al1-xSix phase transformation can be considered as a structural transition of fee Si to tetrahedral Si inside of large Si clusters embedded in the AI-rich matrix.