In a previous study, Mg-Sn-Zn-based alloys showed insufficient structural stability at elevated temperatures. In order to improve the castability and corrosion resistance 2.1%wt Al was added to the Mg-5.6%Sn-4.4%Zn base alloy. At the as-cast condition, SEM micrographs indicate a very fine microstructure (Dendritic Arm Spacing-DAS-smaller than 17 mu m). The study focuses on precipitation hardening, phase formation and structural stability, during the aging of solution treated samples at elevated temperatures. After solution treatment and aging at 225 degrees C, Vickers hardness measurements show that this alloy maintains a constant increase of 30% in hardness for periods of up to 32 days. EDS (SEM & STEM), XRD, and Auger characterization methods were applied to identify the phases presented in the alloy. There is no evidence for the presence of the deleterious gamma-Al-12 Mg-17 phase. SAXS measurement and STEM micrographs reveal very fine precipitations (less than 100 nm) after 32 days of aging, along with homogenously distributed larger precipitations (up to 500 nm).