Cooled exhaust gas recirculation (EGR) is widely used in diesel engines to control engine-out NOx (oxides of nitrogen) emissions. A portion of the exhaust gases is re-circulated into the intake manifold of the engine after cooling it through a heat exchanger. EGR cooler heat exchangers, however, tend to lose efficiency and have increased pressure drop as deposit forms on the heat exchanger surface due to transport of soot particles and condensing species to the cooler walls. In this study, condensation of water vapor and hydrocarbons at the exit of the EGR cooler was visualized using a fiberscope coupled to a camera equipped with a complementary metal oxide semiconductor (CMOS) color sensor. A multi-cylinder diesel engine was used to produce a range of engine-out hydrocarbon concentrations. Both surface and bulk gas condensation were observed with the visualization setup over a range of EGR cooler coolant temperatures. No condensation of water or hydrocarbons was observed above 40°C coolant temperature. For the high hydrocarbon concentration condition, visual observation of onset of condensation correlated with a significant increase in outlet nucleation mode particle concentration. Since diesel engine exhaust consists of several hydrocarbon (HC) species, n-hexadecane (C₁₆H₃₄) was selected as a representative hydrocarbon species. Condensation of n-hexadecane and water vapor to the cooler walls was analyzed using a simple model. In addition surface condensation flux was calculated for several fuel-based hydrocarbon species that may be present in the feed gas to the EGR cooler. Based on this analysis, it was concluded that heavier HC species are more likely to condense at the coolant temperatures (30-90°C) typically encountered in diesel engine EGR coolers. Surface condensation of water vapor and hydrocarbons can have a significant impact on the deposit morphology and, thus, fouling of the EGR cooler. Nucleation mode formation downstream of the EGR cooler below 40°C coolant temperature was found to be dependent on the saturation ratio.