Corrosion failure in most organic coating/substrate systems initiates as a local event that may originate at a chemical or physical heterogeneity within the coating itself or possibly in the substrate. The ability to detect, map the location, and make quantitative bl situ measurements of coating heterogeneities will help identify the source of failure (ie., coating chemistry, method of application, cure schedule, etc.) and provide insight into the mechanisms of coating degradation. This study examines the efficacy of local electrochemical impedance mapping/spectroscopy for the examination of local corrosion events on coated steel substrates. Various types of intentional local heterogeneities were successfully detected with a five-electrode system that utilizes a split microreference electrode. These heterogeneities included chemical defects within the coating such as absorbed oil and physical defects such as subsurface bubbles, underfilm salt deposits, pinholes, and directional underfilm corrosion.