The mechanical properties of high volume fraction SiC-particle reinforced Al-based metal matrix composites (MMCs) produced by means of pressurized liquid metal infiltration (squeeze casting) are shown to be triggered by matrix alloying and heat treatment procedures. It is distinguished between the effect of those alloying elements that only act on matrix strengthening, leaving the interface unaffected, and those alloying elements that interact with both (i.e. Mg). Among the first category a further sub-division is made between pure solid solution and precipitation hardening elements (i.e. Zn and Cu, ZnMg, respectively). In particular, this study addresses the effect of alloying and age hardening for AlCu3 and AlZn6MgI as well as the specific role of Mg additions to Al/SiC MMCs on interface microstructure formation, mechanical properties and fracture mode. For instance, it is shown that single additions of Mg catalyse the formation of Al4C3 whereas additions of Cu as well as (Zn + Mg) provide opportunities to enhance the composites' strength