Solid oxide fuel cells (SOFCs) are electrochemical devices that rely on ion migration through a solid-state electrolyte to oxidize fuel and produce electricity. The present study employs Fourier transform infrared spectroscopy to quantify the exhaust of an SOFC operating with fuel flows of methane over Ni/YSZ cermet anodes and butane over Ni/YSZ and Cu/CeO2/YSZ cermet anodes. Data show that hydrocarbon fuels can participate in a variety of different reactions including direct electrochemical oxidation, various reforming processes, and surface-catalyzed carbon deposition. These findings have direct consequences for assessing the environmental impact of SOFCs in terms of the exhaust discharged from devices operating with common hydrocarbon fuel feeds. In the work presented below, a measure of fuel oxidation efficiency is found by comparing the partial pressure of CO2 (PCO2) in the SOFC exhaust to the partial pressure of CO (PCO). The fuel anode combination with the largest PCO2/PCO ratio is the C4H10 over Cu/CeO2 combination (0.628 ± 0.016). The CH4 over Ni cell type has the second highest ratio (0.486 ± 0.023). The C4H10 over Ni cell type gives a ratio of 0.224 ± 0.001. Attempts to balance the carbon content of the fuel feed and exhaust lead to predictions of SOFC fuel oxidation mechanisms.