One of the most attractive features of solid oxide fuel cells is their flexibility for fuels so that internal reforming and/or simple external reforming may be possible. In this study, equilibria in various possible fuel gases are considered, and C―H―O diagrams are constructed. Power generation characteristics for these fuels are measured and compared with those for simulated reformed gas in equilibrium compositions. We have succeeded to demonstrate direct-alcohol SOFCs for e.g. methanol, ethanol, and propanol. Mixtures of CH4 and CO2 are used as simulated biogas, and iso-octane (C8H18) and n-dodecane (C12H26) are highlighted as simulated gasoline and kerosene, respectively. Influence of fuel impurities on power generation characteristics is also discussed.