This paper aims at quantifying the wettability of the solid carbon fuel by the molten carbonate in a Direct Carbon Fuel Cell (DCFC), to understand its effect on DCFC performance. The wetting behavior of carbon (graphite) under different conditions is observed and analyzed. Direct observation and the time evolution of open circuit potentials (OCP) measured in this work suggest that the wetting phenomena are driven not only by capillary forces but also by the electrochemistry of the reverse Boudouard reaction. A semi-quantitative model is proposed to explain the wetting dynamics of carbon. It shows that the progress of wetting is determined primarily by the production of CO at the carbon surface due to the reverse Boudouard reaction, and the OCP is therefore determined by the rate of the reverse Boudouard reaction as well as the mass transfer limitation of CO removal from the electrode. Consequently, the OCP is a function of operating temperature, ambient gas composition, and the immersed length of carbon electrode. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.022210jes] All rights reserved.