To study localized corrosion mechanism of 2 x x x-series Al alloys containing theta' and S precipitates, the electrochemical behaviors of a three-electrode coupling system of alpha(Al) substituting for alloy base, simulated bulk theta' and S in 4.0% NaCl solution at pH 6.1 have been investigated. The potential of theta' is positive to that of alpha(Al) and S, indicating that theta' acts as cathodic zone in the alloy. S is main anodic phase, and the anodic dissolution of S will cause its pitting corrosion. However, during the corrosion of S, Mg is preferentially dissolved, resulting in noble Cu-rich remnants, which makes its potential move to positive direction with immersion time. As a result, corroded S becomes cathodic to alpha(Al), leading to anodic dissolution of alpha(Al) and pitting corrosion of the alloy base at adjacent periphery of S precipitate. (c) 2004 Elsevier B.V. All rights reserved.