To ensure the proper performance of a hydrocarbon reformer, the fuel and reforming agents should be mixed properly within a short time to suppress gas-phase ethylene production, a well-known deposit precursor that could lead downstream catalyst failure. To examine potential interactions between reactant mixing and gas-phase reaction kinetics in the mixing region, coupled computational fluid dynamics (CFD)-kinetics simulations are performed for autothermal reforming. n-heptane is selected as a representative hydrocarbon fuel. The simulations show clear Negative Temperature Coefficient (NTC) behavior within the temperature range of 450-625 degrees C. At temperatures below the NTC region, the gas-phase reactions are rapid and highly exothermic, making the impact of mixing substantial. Ethylene is produced via a partial oxidation mechanism and is enhanced when the local O/C ratio exceeds the global value. Above the NTC region, ethylene is primarily produced from slower pyrolysis reactions, and then efficient mixing slightly suppresses the ethylene yield. The results suggest the counterintuitive conclusion that mixing at higher temperatures actually suppresses the undesirable reactions. Copyright (c) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.