The use of exhaust gas recirculation (EGR) in internal combustion engines has significant impacts on engine combustion and emissions. EGR can be used to reduce in-cylinder NOx production, reduce fuel consumption, and enable advanced forms of combustion. To maximize the benefits of EGR, the exhaust gases are often cooled with liquid to gas heat exchangers. However, the build up of a fouling deposit layer from exhaust particulates and volatiles results in the decrease of heat exchanger efficiency, increasing the outlet temperature of the exhaust gases and decreasing the advantages of EGR. This paper presents an experimental data from a novel in situ measurement technique in a visualization rig during the development of a 378 μm thick deposit layer. Measurements were performed every 6 hrs for up to 24 hrs. The results show a nonlinear increase in deposit thickness with an increase in layer surface area as deposition continued. Deposit surface temperature and temperature difference across the thickness of the layer was shown to increase with deposit thickness while heat transfer decreased. The provided measurements combine to produce deposit thermal conductivity. A thorough uncertainty analysis of the in situ technique is presented and suggests higher measurement accuracy at thicker deposit layers and with larger temperature differences across the layer. The interface and wall temperature measurements are identified as the strongest contributors to the measurement uncertainty. Due to instrument uncertainty, the influence of deposit thickness and temperature could not be determined. At an average deposit thickness of 378 μm and at a temperature of 100 °C, the deposit thermal conductivity was determined to be 0.044 ± 0.0062 W/m K at a 90% confidence interval based on instrument accuracy.