The effects of boron and strontium interactions on the eutectic silicon in hypoeutectic Al–Si alloys have been studied. Samples were prepared from an Al–10 mass%Si base alloy with different Al–B additions, alone and in combination with strontium. In alloys containing no strontium, boron additions do not cause modification of the eutectic silicon, while in strontium containing alloys, boron additions reduce the level of modification of the eutectic silicon. Thermal analysis parameters and eutectic silicon microstructures were investigated with respect to the Sr to B ratio. In order to modify the eutectic silicon, a Sr/B ratio exceeding 0.4 is required.