The use of exhaust gas recirculation (EGR) in internal combustion engines has significant impacts on combustion and emissions. EGR can be used to reduce in-cylinder NOx production, reduce emitted particulate matter, and enable advanced forms of combustion. To maximize the benefits of EGR, the exhaust gases are often cooled with on-engine liquid to gas heat exchangers. A common problem with this approach is the build-up of a fouling layer inside the heat exchanger due to thermophoresis and condensation, reducing the effectiveness of the heat exchanger in lowering gas temperatures.Literature has shown the effectiveness to initially drop rapidly and then approach steady state after a variable amount of time. The asymptotic behavior of the effectiveness has not been well explained. A range of theories have been proposed including fouling layer removal, changing fouling layer properties, and cessation of thermophoresis. In an effort to investigate this phenomenon, an EGR cooler visualization rig has been constructed. This rig incorporates an optically and infrared transparent access window, allowing for measurement of key deposit layer variables in-situ. These variables coupled with heat flux measurements will allow for the calculation of thermal properties of the deposit layer as a function of deposit thickness, engine condition, and emissions levels.Initial results have shown a linear growth in deposit layer thickness over a 24 hour deposition interval with a nonlinear change in deposit surface temperature. These results potentially identify a change in properties of the deposit layer. In addition, the surface area ratio of the deposit layer increases significantly as the deposit thickness increases and could be significant for heat transfer.This paper describes the visualization rig, methodology for in-situ measurements, and initial results.