An increased interest in using hydrocarbons in solid oxide fuel cells for the production of power has led to research into operation on synthesis (syn) gas, a mixture of hydrogen and carbon monoxide. Hydrocarbons are typically reformed, either internally or in an external reformer prior to the fuel cell, producing syngas with various H-2:CO ratios depending on the hydrocarbon used. This paper examines the effect of varying the H-2:CO ratio with respect to C-1 to C-4 steam reforming reactions and additionally a mixture containing a higher ratio of carbon monoxide. It was found that there was no significant relationship between cell performance and H-2 :CO ratio when a high feed rate was employed. For low flow rates, however, the high carbon monoxide concentration resulted in a significant decrease in cell performance. It was determined that this was caused by reversible carbon deposition as opposed to a decrease in carbon monoxide reactivity. (C) 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.