Die cast Mg-Al alloys in the AM, AS, AZ and AE series have been corrosion tested by salt spray and immersion in 5% NaCl solution. The corrosion rate increases rapidly when the Al-content decreases below about 4% for all alloy systems. AE alloys exhibit a high resistance to localized attack because the Al-rich coring along grain boundaries appears to act as an efficient barrier against pit propagation in these alloys. The role played by different intermetallic phases found in the Mg-Al alloy systems is evaluated, based on electrochemical data obtained from synthetically prepared phases. Fe-rich phases are particularly detrimental, but Al-Mn phases with a low Al/Mn ratio may also have a high cathodic current output. The phases Mg2Si and Al4MM (MM = Misch Metal) appear to be harmless from a corrosion point of view.