Solid oxide fuel cells (SOFCs) can operate by using various fuel species. Pressurized SOFCs with gas/steam turbine(s) may achieve higher power generation efficiency as hybrid or triple-combined power generation systems. In this study, fuel gas composition is systematically investigated by thermochemical equilibrium calculations on the anode side of SOFCs, pressurized up to 30 bar over a wide temperature range, up to 1000 degrees C. Since conventional hydrogen-containing fuel gas consists mainly of carbon, hydrogen, and oxygen, high-pressure C-H-O equilibrium diagrams are numerically obtained. It is revealed that the carbon deposition region contracts in the hydrogen-rich area and expands in the oxygen rich area with increasing total pressure. The molar fraction of each gas component, described in such C-H-O diagrams, also depends on the total pressure. The theoretical open circuit voltage (OCV) increases by pressurization. The effect of nitrogen in high-pressure SOFC fuels is also considered, which is important especially for air-blown coal gas. The minimum amount of H2O, O-2, and CO2 required to prevent carbon deposition in steam reforming, partial oxidation, and CO2 (dry) reforming, respectively, is also derived up to 30 bar. The high-pressure C-H-O diagrams are also applicable to various high-temperature/high-pressure energy systems such as solid oxide electrolyzer cells (SOECs) and reversible fuel cells. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.