Natural gas was reformed with anode off-gas of a solid oxide fuel cell (SOFC) in a laboratory reactor using model gases. Nickel and precious metal catalysts were tested at 650 degrees C and 700 degrees C inlet temperature. The oxygen to carbon (O/C) molar ratio of the inlet gas was varied between 0.3 and 2.0 to find out the limit O/C ratio for carbon formation. Two natural gas compositions were used: low purity Danish gas and high purity Russian gas. The experimental limit was found to be 0.9-1.25 for nickel catalyst and 0.5-0.75 for precious metal catalyst. The limit was higher with Danish gas than with Russian gas in the experiments with nickel catalyst but this effect was not observed with the precious metal catalyst. A good correlation between experimental results and thermodynamic calculations was found using the principle of actual gas: the calculations estimated the O/C limit ratio for nickel at 1.0 and for precious metal catalyst between 0.5 and 0.7. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.