Strontium is becoming an important modifier of Al-Si alloys, being increasingly preferred over sodium and antimony since it is easier to handle and has less detrimental environmental effects. Additionally, strontium is easy to add to aluminum based alloys as this system presents several intermetallic phases, facilitating the fabrication of master alloys. When Sr is added in small quantities to Al-Si alloys, their eutectic structure changes from thick flakes to fine interconnected fibbers, consequently improving their mechanical properties. Although Sr is added in relatively small amounts, segregation during processing may lead to the formation of several intermetallic phases that affect the overall performance of Al-Si alloys. Thus, it is of interest to understand the phase stability of the Al-Si-Sr ternary system. As a first approach, we have currently developed the thermodynamic model for the Sr-Si system.