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The molten carbonate fuel cell (MCFC) is an energy-producing electrochemical system which is expected to become commercial in the near future. High operating pressure, ca. 12-25 atm, is required to develop a highly efficient MCFC plant system combined with a gas turbine or coal gasifier system. However, pressurized performance over 10 atm has not been measured. It is important to study the performance of the MCFC at higher pressures than 10 atm from the viewpoint of developing a high-pressure system. In addition, an operating technique for avoiding carbon formation in the anode side has not been evaluated.The most important cause of lifetime limitation is dissolution of the cathode and precipitation of Ni in the electrolyte. 2-10 Operation at pressures over 10 atm accelerates the dissolution and shortens cell life. From such viewpoints an economical investigation must be conducted including optimal cell life time and performance in such a high-pressure system. Development of the estimating model of cell performance would be very useful for such studies.In this paper, as the first step of this study, we experimentally clarify cell performance using Li/K and Li/Na carbonates as the electrolyte at wide pressure ranges between 1 and 45 atm with an anode gas composition avoiding carbon formation.Experimental Carbon deposition study.-Carbon deposition in pressurized operation of the MCFC has to be avoided because it can lead to plugging of fuel gas passages. The partial pressure of H 2 O in the fuel gas stream can control it. The optimum partial pressure of H 2 O must be determined to operate a MCFC at high pressure over 10 atm without carbon deposition problems.Thermodynamic calculations of carbon deposition boundaries can provide a first criterion for determining whether we can expect carbon deposition under any specific conditions. We calculated the carbon deposition boundaries for the fuel gas composition of the MCFC and determined the amount of H 2 O compared with some experimental data on carbon deposition using a dummy cell. The dummy cell made of aluminum-coated SUS310 containing nickel powder as a catalyst of the shift reaction in the fuel gas flow channel was used for the reactor. The reactor was heated to 823 K supplying N 2 gas, and then fuel gas was supplied to the reactor for 24 h. Carbon formation was detected by visual observation of the surfaces after disassembly.Analysis of Cell performance.-The cell was assembled using state-of-the art components of the IHI (Ishikawajima-Harima Heavy Industry), the specifications of which are shown in Table I. Mixtures of 70 mol % Li 2 CO 3 and 30 mol % K 2 CO 3 , and 60 mol % Li 2 CO 3 and 40 mol % Na 2 CO 3 were used as the electrolyte. The effective cell area was 110 cm 2 and the thickness of the matrix was 0.092 cm. No additive to the matrix was used to reduce the NiO solubility in the electrolyte. Single cells were operated at 923 K and at several pressures up to 44.5 atm. The load current density was fixed at 150 mA/cm 2 with 60% fuel utilization. Standard gas ...