Individual Sr and B additions to Al-Si alloy melts fail to provide either grain refinement or modification. Sr and B must be used together to achieve both features for high-quality sound castings. Sr and B solutes in addition to insoluble SrB6 particles are introduced to the melt when Sr and B are used together. SrB6 particles and B solutes, which form the AlB2 particles later during solidification, are believed to be responsible for the marked improvement in grain refinement in the co-addition practice. Sr atoms in liquid solution, on the other hand, are adsorbed on the surface of the Si plates leading to twinning once the melt cools to the eutectic solidification temperature. A series of experiments have shown that a very fine grained matrix and a fine fibrous eutectic are readily obtained when the AlSi7Mg0.3 alloy is inoculated with 100 ppm B and 150 ppm Sr.