Cooled exhaust gas recirculation (EGR) is effective in the reduction of in-cylinder formation of oxides of nitrogen (NOx). However, at conventional in-cylinder high temperature diesel combustion high load operating conditions, the engine-out exhaust emissions of particulate matter, which consists of smoke, soot and soluble organic fraction, tend to increase. As the EGR is applied at medium to high loads, therefore, the small carbonaceous particles and heavy hydrocarbons can deposit on the cool surfaces of EGR coolers mainly through a complex combination of thermophoresis, condensation, diffusion and turbulent impaction. Consequently, an insulation layer develops on the cooler surfaces and deteriorates EGR cooler function. As the deposits build up, the cooler effectiveness decreases and pressure drop increases thereby increasing the intake charge temperature. Consequently, the in-cylinder formation of NOx increases. In the present study, two commercially available “fin-type” EGR coolers of differing size and design configuration were aged at freeway cruise operating conditions and the performance compared to that of a more traditional “shell-and-tube” EGR cooler. Using a modern V8 turbocharged, direct-injection common-rail diesel engine coupled to a dynamometer, raw EGR was applied for a specified time followed by a 30 minute shut down. This sequence was repeated until the EGR coolers attained an accumulated run-time of 50 hours. The cooler effectiveness, pressure drop and recovery characteristics, EGR flow rate and engine-out NOx, Carbon Monoxide, smoke and total hydrocarbons were investigated.