Whisker carbon formation in the steam reforming of biomass gasification gas was studied in a laboratory scale reactor using two commercial nickel catalysts, precious metal catalyst and inert materials. The synthetic feed gas contained ethylene, tar model compounds and H2S as impurities. Whisker carbon was formed below the reaction temperature of 700 degrees C on an calcium-doped nickel catalyst and below 850 degrees C on an undoped nickel catalyst when the feed gas contained no sulfur. With the addition of more than 50 ppmv of H2S in the feed gas, the whisker carbon formation was inhibited. Thermodynamic calculations were carried out to estimate the upper limit temperature for the whisker carbon formation but the calculations did not correlate well with the experimental results. One of the probable explanations for this was the high concentration of unsaturated C2+ hydrocarbons in the feed gas.