By means of scanning electron microscopy, electron probe microanalyses, and differential scanning calorimetry, 21 alloys in both as-cast and annealed states were investigated to study the phase equilibria and phase transformations of the Al-Sc-Si system in the Al-rich corner. Based on the observed microstructure, the solidification paths of the as-cast alloys were analyzed. In addition, the phase equilibria of the Al-Sc-Si system at 500 A degrees C were determined and phase transition temperatures for representative alloys are measured by means of DSC. For the sake of providing missing thermodynamic data, the enthalpies of formation at 0 K for the compounds ScSi, Sc5Si3, and tau (AlSc2Si2) were obtained by first-principles calculations. Based on the data from the present work and literature, the thermodynamic descriptions of the Sc-Si and Al-Sc-Si systems were developed. The Al-Sc-Si phase diagram in the entire composition range and the solidification characteristic in the Al-rich corner were then calculated. It is shown that the predicted solidification paths could describe the experimental observations reasonably. Based on the thermodynamic description obtained in this work, solution heat-treated and aged conditions of some Al-Sc-Si alloys are discussed, which suggests that the validated thermodynamic descriptions of this work could be helpful for the microstructure design of the Al-Sc-Si alloys.