When Mg or Mg-alloys are reinforced with Ti 2AlC - using a simple pressureless melt infiltration method - the result is nanocrystalline, nc, Mg-matrix composites, with outstanding mechanical properties. As an added bonus, the nc Mg-matrix is extraordinarily thermally stable. When AZ61 is used to infiltrate the Ti 2AlC preforms, ultimate tensile stresses of 800 MPa are achieved. The reasons that lead to the formation of the nc-Mg are as of yet not understood. In this study, the different composites' microstructures are investigated by X-ray diffraction, scanning and transmission electron microscopy. A correlation was found between the presence of oxygen and the nc Mg grains. Nanometer sized Mg grains were also found in between some of the Ti 2AlC layers.