The mechanism of trench formation next to cathodic intermetallic compound particles (IMC) on bare polished AA 2024-T3 was studied in situ using confocal laser scanning microscopy. Trenches formed at the interface between the matrix and some IMC of the Al-Cu-Mn-Fe and Al-Cu types in all electrolyte solutions studied, including 0.1 MNa2SO4 + 0.005 M NaCl with pH adjusted to 3, 6, and 10 as well as in near-neutral 0.5 M NaCl (pH 6). The trenches in the acidic solution were narrower, and not every cathodic IMC examined developed trenches. Two models for trench formation are compared in their ability to rationalize the experimental observations. The alkaline model ascribes the trenching completely to the effects of local pH increase and is unable to rationalize trenching at low pH, the dependence of trenching on particle type, and the effect of chloride ion content on trenching rate. An anodic trench model combines the influences of IMC cathodic kinetics, galvanic coupling, and anodic dissolution of the matrix, and particle/matrix metallurgy on the trenching behavior. The latter model is able to rationalize the effects of chloride content, bulk pH, as well as the observation that not all particles of a given type undergo trenching. (C) 2004 The Electrochemical Society.