A study of the effect of strontium on some solidification parameters, such as eutectic nucleation temperature, eutectic growth temperature, eutectic undercooling temperature and eutectic undercooling time, has been carried out using thermal analysis for a composite reinforced with 15 vol. % SiCP and, for comparison, for an A356 aluminum alloy. The composite is prepared by the melt stirring technique with a SiC particle size of 38 μm. Thermal analysis results show that the presence of SiCP in the unmodified A356 aluminum alloy increases the eutectic growth temperature (TE) and the eutectic nucleation temperature (TNucl); on the contrary, SiCP decreases the eutectic undercooling temperature (θ) and the eutectic undercooling time (tE). These phenomena suggest that SiC particles give favorable conditions for the growth of eutectic silicon. On the other hand, the modification with strontium of the composite material, although showing basically the same effect on the eutectic parameters as the one described for the A356 aluminum alloy, brings about certain differences due to the presence of the SiC particles. Microstructural analysis shows that the eutectic structures in the composite are coarser than those of the matrix alloy and they do not have the classic fibrous eutectic shape obtained in the matrix alloy. For the matrix alloy, when the Sr concentration increases beyond the quantity required to obtain a well-modified structure, the eutectic structure suffers a gradual coarsening or a reversion from fine fibrous silicon to coarser silicon; subsequently, when the Sr concentration is higher than 0.068%, Al2Si2Sr particles are produced. In the composite material there is also a gradual coarsening of the eutectic structure, although the appearance of Al2Si2Sr particles is just seen when the Sr concentration reaches 0.106%.