The fundamental thermodynamic relations governing the behavior of a fluid phase residing within the interstices of a porous solid phase are reviewed. The relationships are applied to the problem of electrolyte distribution among the components of the molten carbonate fuel cell. This distribution is of paramount importance in determining cell electrochemical performance. In addition, the function of the electrolyte structure, or “tile,” as a gas impermeable partition between the anode and cathode environment is considered.