The mechanism for the oxidation of solid carbon in a fuel cell made as a hybrid between a molten carbonate fuel cell and a solid oxide fuel cell, known as a hybrid direct carbon fuel cell (HDCFC), was investigated: Fuel cell performance was measured by electrochemical impedance spectroscopy (EIS) and using current-potential-power density curves (I-V-P) between 700 to 800 degrees C. The impacts of the gas species introduced at the cathode (air vs. pure O-2) and at the anode (pure N-2, pure CO2, and mixed N-2-CO2) were investigated, as well as the influences of temperature and anode gas flow rate. The majority of the impedance data could be modeled using an equivalent circuit consisting of a resistor (Rs) in series with three resistor-constant phase element units (RQ, in parallel), depending on anode gas atmosphere. An explanation was proposed for each impedance element, and the literature relating to impedance data acquired for carbon-carbonate mixture in a DCFC anode were discussed. By varying of the anode gas mixtures between pure N-2 and pure CO2, together with variations in their flow rates, it was suggested that CO2 is a chemically active species which is not electrochemically active, a chemical intermediate in the oxidation of solid carbon in such a HDCFC. (C) 2013 The Electrochemical Society.