In this study, the effects of inlet conditions on the performance of a natural gas auto-thermal reforming reactor loaded with a commercial monolith catalyst are investigated. The reactor has a hydrogen production capacity of 1.5 kW and, is a part of a fuel processor, applicable in a residential-scale fuel cell system. Experimental, kinetic and equilibrium results are all presented. The experimental data were input into commercial software, Aspen HYSYS (ver.8.8). Equilibrium state calculations are based on the maximization of entropy. Monolith catalyst performance is consistent with thermodynamics, especially for lower oxygen feeding. The kinetic is also run into HYSYS and the results are in harmony with the experimental findings. The effects of the operating parameters, namely the oxygen-to-carbon ratio, the steam-to-carbon ratio and the reactor inlet temperature, on the hydrogen yield, fuel conversion, efficiency, and compositions are discussed experimehtally and theoretically. The main impact among the parameters that affect the monolith performance is determined as the oxygen-to-carbon ratio. The favourable operating conditions are determined as inlet temperatures of 400 degrees C-550 degrees C, the steam-to-carbon ratio of 3.0, and the oxygen-to-carbon ratio of 0.5 with the hydrogen yield of 2.32-2.46, fuel conversion of 90%-96.5% and the efficiency of 67-72%. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.