The beta-phase Mg₁₇Al₁₂ influences high temperature strength as well as corrosion properties of Mg-Al alloys. Mg₁₇Al ₁₂ develops during casting and its morphology depends also on heat treatments and processing. Normally the beta-phase, its morphology and distribution is determined by standard metallographic methods leading only to a two dimensional picture. Synchrotron-radiation based microtomography (SRμCT) is able to give a 3D image of the microstructure and is used here to characterize the distribution of Mg₁₇Al₁₂ in as-cast and extruded AZ91D. The synchrotron radiation based results have been supplemented by XRD and methods of metallography.