Low carbon steel substrates were covered with a gamma-glycidoxypropyltrimethoxysilane (gamma-GPS) modified epoxy layer. Electrochemical impedance spectroscopy and attenuated total reflection infrared spectroscopy were used to investigate the barrier properties of the coating. The resistance of the samples to ion transport processes along the epoxy/steel interface was determined by in situ scanning Kelvin probe (SKP) measurements of the interface potential. X-ray photoelectron spectroscopy studies helped to analyze the resulting ion distribution on the substrate surface. The application of gamma-GPS resulted in a reduction of the interfacial water activity and temporarily stabilized the polymer/steel interface. In contrast to the experiments with the unmodified epoxy coating, interfacial ion transport processes were verifiable with the SKP. After some days of sample exposure to humid air, the stabilizing effect of gamma-GPS diminished and SKP potential profiles had to be recorded in dry atmosphere to identify the electrolyte front position, zones of cation and anion separation, and areas of local corrosion damage at the interface. This approach seems to be generally promising to analyze the degradation state of polymer/oxide/metal interfaces after long-term exposures in humid air. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3525240] All rights reserved.