The wetting behavior of the Au electrode under molten carbonate fuel cell (MCFC) operating conditions was investigated using a lab-scale cell with optical instrumentation. The general trends which were found were smaller contact angles, (i) under polarization than at open circuit, (ii) under an oxidizing atmosphere than under a reducing atmosphere, (iii) at high temperature rather than at lower temperature, and (iv) in Li-K carbonate than in Li-Na. Contact angles as a function of potential exhibit a dependence which resembles electrocapillary curves in aqueous solution, with the zero-charge (minimum wetting) potential and the open-circuit potential (OCP) being different, especially under a reducing atmosphere. The effect of electrolyte composition on the contact angle is interpreted in terms of activity and surface adsorption of oxides. The effect of polarization on wetting is analyzed from two different viewpoints, near OCP the electrocapillary theory is applied, while at high polarization the effect of migration is invoked. The inferior performance of Li/Na cells, compared to Li/K cells, at temperatures below 600degreesC, is analyzed from the viewpoint of capillary effects, and it is shown that these effects may explain the observed anode behavior and, at least in part, cathode behavior as well. (C) 2003 The Electrochemical Society.