Selective laser melting (SLM) is used to produce a SiC reinforced aluminum metal matrix composite (AMMC, Al-12Si plus 10vol% SiC) with laser energy densities (Ep) between 20 and 80Jmm(-3). Microstructural analysis shows that at lower energies, SiC is present in the Al-12Si matrix; however, at higher energies there is a distinct lack of SiC particles and the extensive formation of Al4C3 needles and primary Si particles. XRD analysis confirms a decrease in the volume of SiC and an increase in the amount of Al4C3 and primary Si with increasing . This indicates that a reaction occurs between the Al and SiC during SLM. The underlying mechanism is attributed to the selective absorption of laser energy into the SiC particles, causing regions of extremely high temperatures. The formation of the reaction products cause errors in the theoretical density calculations. Therefore, X-ray micro tomography (XMT) is used to independently measure the relative density of the samples with a peak relative density approximate to 97.4%, which is much higher than that (relative density approximate to 93%) measured using the Archimedes method.