Epoxy-coated AA2024-T3 samples were exposed to solutions of specific chloride concentration and pH. Underfilm corrosion processes were monitored in situ with confocal laser scanning microscopy. In near-neutral solutions, pitting at both intermetallic compounds and within the alloy matrix was observed during coating blister formation and growth. Evidence for anodic undermining/corrosion product wedging was obtained. In addition, general corrosion of the matrix led to a strong roughening of the surface. In acidic NaCl solutions, localized corrosion was observed at the original blister site. Thereafter, general attack on the Al-matrix near the original blister was more important than intermetallic particle corrosion. In acidic sulfate solutions, the coating started to fail at many locations on the sample at the same time due to attack of the coating and loss of adhesion at the interface. This more global failure is in agreement with electrochemical impedance measurements and the observation of numerous blisters on large-area panels. (c) 2007 The Electrochemical Society.