Free corrosion immersion experiments were conducted on a commercial airframe material, Al 2024-T3 (UNS A92024), in 0.5 M sodium chloride (NaCl) solution to investigate the role of microconstituents in pitting corrosion. The alloy was found to contain numerous constituent particles (> 300,000 per cm2 [> 2 million per in.2]), and pitting corrosion essentially was attributable to these particles. Two types of constituent particles were identified. Each type played a different role in inducing pitting corrosion. Particles containing Al, Cu, Fe, and Mn acted as cathodes and promoted matrix dissolution at their periphery. Particles containing Al, Cu, and Mg showed anodic behavior and dissolved with preferential dissolution of Mg and Al. It was evident that individual particle-nucleated pits coalesced, laterally and in depth, to form larger pits. Tests conducted in deionized water indicated that only the Al-Cu-Mg-containing particles dissolved to produce localized corrosion damage at the surface. Microconstituent-induced pitting was found to depend upon solution pH.