Manganese is used to remove iron from molten magnesium alloys in order to produce high purity alloys. A study of manganese-containing particles in the magnesium alloys is therefore of technological significance with regard to the corrosion properties of Mg-Al-based alloys. TEM/SEM investigations of the as-cast microstructure and the corrosion morphologies revealed that the corrosion resistance of the alloys studied is dependent on (1) the distribution and types of small Mn-containing particles where corrosion pits can form to initiate corrosion fissures, and (2) the barrier effects for the propagation of fissures in the alloys. Two types of manganese-containing phases were found in the alloys. Type I is a flower-shaped phase, and Type II is of equiaxed or short-bar morphologies. The type II particles have a lower Al/Mn ratio and a much higher cathodic reaction rate than the Type I phase. The Type II particles may provide sites to form pits and initiate corrosion fissures. The segregated regions of higher Al content adjacent to the grain boundaries and the β-Al12Mg17 particles may act as barriers to the propagation of corrosion fissures.