Magnesium has gained considerable interest as a structural material for automotive and aerospace applications due to its low density and high specific strength. The use of magnesium alloys in engineering applications is, however, mainly limited by their unsatisfactory surface properties and their poor corrosion resistance. In recent years, significant improvements have been made in achieving a better corrosion resistance particularly by reducing the contents of impurities such as Fe, Cu, and Ni. Intermetallic phases, a result of the casting process, play an important role in the corrosion process. The role of these intermetallic phases has been addressed in a number of papers [1–3]. The role of aluminum in the atmospheric corrosion of magnesium alloys has less addressed in the literature [4]. Esmaily et al have shown recently that the rate of corrosion of magnesium alloys decreased when increasing the Al content. However these experiments were performed on commercial magnesium alloys produced high pressure die casting under constant RH exposure at 90% RH. In our work the corrosion behavior of binary Mg-Al alloys as well as that of commercial magnesium alloys with different aluminum contents have studied both under cyclic corrosion exposure in the laboratory and in field exposures in France and in Vietnam. Analyses of corrosion products have also been conducted. Additional in-situ measurements have been performed in order to better understand the atmospheric corrosion mechanisms of Mg alloys. An important decrease in the maximum pit depth can be observed after different laboratory exposures when increasing the aluminum content in the material. Similar results have been obtained after 9 months exposure in Field. Hence, from these results it is obvious that an increase of the aluminum concentration in Mg-Al alloys decreased the corrosion rate of the alloy. This lead in lower mass loss and less deep corrosion attacks in the magnesium grains. The analyses of corrosion products formed after the cyclic corrosion tests and field exposures revealed the presence of Magnesium carbonate and MgCO3(OH)4,4H20, respectively. In both cases magnesium hydroxide (Mg(OH)2 was also found in the corrosion products. The results will be discussed in relation with the microstructure of the cast materials.