This paper presents experimental study on catalytic autothermal reforming (ATR) of natural gas (NG) for hydrogen (H-2) production over sulfide nickel catalyst supported on gamma alumina. The experiments are conducted on a cylindrical reactor of 30 mm in diameter and 200 mm in length with "simulated" NG of different composition under thermal-neutral conditions and fed with different molar air to fuel ratio (A/F) and molar water to fuel ratio (W/F). The results showed that reforming performance is significantly dependent on A/F, W/F and concentration of C2+ hydrocarbons in inlet fuel. Fuels containing higher C2+ hydrocarbons concentration have optimum performance in terms of more H-2 at higher A/F and W/F but lower conversion efficiency. Good performance for ATR of fuel containing 15%-20% C2H6 can be achieved at A/F = 5-7 and W/F = 4-6, much higher than that for optimum performance of ATR of methane (A/F = 3, W/F = 2-2.5). CO2 in the inlet fuel does not have significant effect on the reversed water-gas shift reaction. Its effect on reforming performance is mainly due to the dilution of inlet fuel and products. (c) 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.