All high-pressure exhaust gas recirculation (EGR) coolers become fouled during operation due to thermophoresis of particulate matter and condensation of hydrocarbons present in diesel exhaust. In some EGR coolers, fouling is so severe that deposits form plugs strong enough to occlude the gas passages thereby causing a complete failure of the EGR system. In order to better understand plugging and means of reducing its undesirable performance degradation, EGR coolers exhibiting plugging were requested from and provided by industry EGR engineers. Two of these coolers contained glassy, brittle, lacquer-like deposits which were analyzed using gas chromatography-mass spectrometry (GC-MS) which identified large amounts of oxygenated polycyclic aromatic hydrocarbons (PAHs). Another cooler exhibited similar species to the lacquer but at a lower concentration with more soot. The authors propose that lacquer deposits form when oxygenated PAHs present in the exhaust condense on the cooler walls subsequently experience nitric acid catalyzed polymerization in the presence of aldehydes. A fourth cooler contained large amounts of oil which likely originated from a failed lubricant seal. In light of these results, recommendations for preventing plugging of EGR coolers are suggested.