It is important to understand the mechanism of corrosion of substrates used for autobody panels to enable materials engineers to predict their cosmetic corrosion performance through properly designed laboratory tests that accurately assess corrosion behavior in actual service. In an effort to better understand the mechanism of corrosion beneath paint films. Armco Research has initiated analytical work to characterize the behavior of materials at various stages of laboratory cyclic corrosion testing.The primary technique utilizes an electron microprobe to produce electron images, elemental x-ray maps and energy dispersive spectra of metallographically prepared corrosion test samples. Details of the experimental technique are presented along with results-to-date on carbon steel and electrogalvanized (EG) samples subjected to cyclic corrosion tests having a high percentage of wet time. Results show underfilm delamination occurs at the substrate-phosphate interface with subsequent intrusion of the electrolyte followed by corrosion. The depth of scribe in the EG samples also has a pronounced effect on the underfilm corrosion mechanism.