The powder preparation of MgSiN2 was studied using several starting mixtures (Mg3N2/Si3N4, Mg/Si3N4 and Mg/Si) in the temperature range 800-1500 degrees C in N-2 or N-2/H-2 atmospheres. The phase formation was followed with TGA/DTA and powder X-ray diffraction (XRD). At 1250 degrees C Mg/Si mixtures did not yield single phase MgSiN2 whereas for Mg/Si3N4 and Mg3N2/Si3N4 mixtures nearly single-phase powders were obtained. The Mg/Si3N4 mixtures yielded MgSiN2 at the lowest processing temperature but the Mg3N2/Si3N4 mixtures yielded the most pure MgSiN2 powder with respect to secondary phases. The main secondary phase detectable with XRD was MgO when starting from Mg3N2/Si3N4 or MgO and metallic Si when starting from Mg/Si3N4 mixtures. When the processing starting from Mg3N2/Si3N4 mixtures was optimised MgSiN2 powders containing only 0.1 wt % O could be prepared. Using XRD the solubility of oxygen in the MgSiN2 lattice was estimated to be at maximum 0.6 wt %. The MgSiN2 powder was oxidation resistant in air till 830 degrees C. The morphology and particle size were studied with the scanning electron microscope (SEM) and the sedimentation method. Two different kinds of morphology were observed determined by the morphology of the Si3N4 starting material.