The interaction between modification with Sr and grain refinement with B was investigated to ensure a very fine grained matrix and a very fine fibrous eutectic in the AlSi7Mg alloy. The grain refinement offered by B is remarkable. B (200 ppm) refines the grain structure of the AlSi7Mg alloy at least 10 times in a wide range of cooling rates between 0·1 and 10 K s−1, whether added before or after Sr. While a weak grain refiner, Sr turns the coarse Si plates and needles into fine fibres. However, the modification capacity of Sr is impaired when there is B in the melt, possibly due to the formation of SrB6 compound. The poisoning in the chemical modification of the eutectic phase in grain refined Al–Si alloys can be circumvented by quench modification. It takes cooling rates in the order of 10 K s−1 to adequately modify the eutectic phase in the Sr free grain refined AlSi7Mg alloy. Si fibres are still the predominant feature at lower cooling rates (1–10 K s−1) when modification of the eutectic phase relies not only on rapid solidification but also on Sr. Coarse Si plates and needles form only at very low cooling rates, well below 1 K s−1. It is fair to claim that 200 ppm B may suffice for a sound structure in casting processes, which involve rapid solidification and thin sections, while Sr is needed for an acceptable modification of the eutectic phase when solidification is relatively slower.