Finned-tube heat exchangers are frequently used in industry for gas-to-liquid heat transfer. For ORC power cycles, they are employed when heat is recovered from exhaust gas streams. The heat exchanger can either be used to directly evaporate the working fluid or to transfer the heat to an intermediate extraction loop. While the literature on heat exchanger design is comprehensive, there is limited information on how the performance of an existing unit can be determined. Performance information is needed to investigate the overall system and to design heat exchangers of a similar type. This paper presents how the thermal performance of an existing finned-tube heat exchanger is determined experimentally. In the experiment, heat is transferred from an exhaust gas stream to a thermal oil. The experimental setup is discussed including the thermal fluid selection process, the components, instrumentation and resultant measurement uncertainty. The heat transfer is calculated individually on either side of the heat exchanger. The purpose of the article is to present a procedure for measuring the performance of a heat exchanger and determining the individual heat transfer coefficients for both sides given limited information from the experiment.