The operation of solid oxide fuel cells with the use of different types of carbon-based fuels (i.e. natural gas, coal gas, etc.) became one of the main topics of SOFC research within the last years, Fuel gases like methane can be steam-reformed or partially oxidized within the SOFC stack, Usually a large amount of steam or air (steam to carbon ratio>2) has to be applied to avoid carbon deposition resulting in a degradation of the anode and a failure of the stack. The influence of the steam to carbon ratio on the performance of planar state of the ail SOFC single cells with conventional nickel/yttria-stabilized zirconia cermet anodes has been investigated using CO/H-2 mixtures as well as CH4/H2O mixtures as fuel gas. The cells were characterized by dc techniques and impedance spectroscopy under realistic working conditions. In the case of CO/H-2 mixtures, a decrease in performance was observed at high CO portions (>90%), whereas the cell performance increased with decreasing S/C ratio using methane as the fuel. In addition, the stability of single cells was investigated. A stable operation using dry methane as fuel (S/C=0) was possible for up to 1000 h without serious degradation,