Age-hardenable aluminum-silicon alloys have attracted increasing attention in recent years, particularly as a result of the demand for lighter vehicles as part of the overall goal to improve fuel efficiency and to reduce vehicle emissions. Among these aluminum cast alloys, the 319-type alloys have become the object of extensive investigation considering their practical importance to the transport industry. All the experimental variables, such as solidification condition, composition, and heat treatment, are known to have an influence on precipitation behavior; precipitation-hardening, however, is the most significant of these because of the presence of excess alloying elements from the supersaturated solid solution which form fine particles and consequently act as obstacles to dislocation movement. The precipitation-hardening behavior of a Sr-modified 319-type alloy containing 0.4% Mg was investigated for this study using transmission electron microscopy. Non-conventional aging cycles were applied so as to evaluate the degree of the improvement in strength potentially obtainable. The results show that the main strengthening phase is theta-Al2Cu occurring in the form of plates; other phases were observed as minor constituents in this alloy, including the binary beta-Mg2Si, the ternary S-CuAlMg2, and the quaternary Q-Al5Cu2Mg7Si7.