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The fine scale microstructure of Al-5083 (H-131) sensitized at 448 K (175°C) for 1, 10, 25, 50, 100, 240, 500, and 1000 hours has been investigated using transmission electron microscopy (TEM) to study the evolution of the b phase (Al 3 Mg 2 ) at grain boundaries and on pre-existing intragranular particles. In fully sensitized Al-5083, the b phase (Al 3 Mg 2 ) forms heterogeneously both at grain boundaries and on pre-existing particles, which are enriched in manganese. TEM observations showed that the grain boundary precipitation of the b phase initially occurs between 0 to 1 hour of aging at 448 K (175°C), and that the b phase grows with a ribbonlike morphology. The grain boundary planes are covered by the b phase after 240 hours of aging. The contribution of microstructure, defects, and environment on the stress corrosion cracking (SCC) behavior is discussed.