The future demand for light weight constructions requires alloy development to achieve an appropriate combination of mechanical properties and corrosion behavior of modern magnesium alloys. Therefore a new class of magnesium alloys based on the Mg-Sn system has been developed. Besides the combination of alloying elements in the new Mg-Sn alloys, the property profile is ruled by the development of microstructure during solidification and subsequent heat treatments. To obtain deeper knowledge of the occurring phases Mg-Sn-Ca and Mg-Sn-Si alloys have been investigated in the F, T4 and T6 condition by means of electron microscopy, XRD and Vickers hardness to determine phases in accordance to the as cast and heat treated conditions. These experiments have been accompanied by the investigation of corrosion behaviour using salt spray test and potentiodynamic measurements.