Recently, there has been a great increase in activity in the field of hydrocarbon fuel cells. A rather neglected area has been that of the thermodynamics of the reactions in the anode compartment. The conditions under which a fuel cell must be operated in order to prevent carbon deposition have been calculated for the system at 500°K under two important sets of conditions. Two literature methods for the calculation of the complex gas phase equilibria were evaluated, and an improved method extending one of these methods is presented. The calculations show that unexpectedly high and constant emf values are possible. Appreciable amounts (ca. 1%) of hydrogen were found to be present at equilibrium over a wide range of fuel compositions at 500°K. The levels of CO partial pressures at equilibrium were found to be about 10−5 atm.