The CH4/O-2/NOx system is investigated in a laboratory-scale high pressure laminar flow reactor with the purpose of elucidating the sensitizing effects of NOx on CH4 oxidation at high pressures and medium temperatures. Experiments are conducted at 100, 50, and 20 bar, 600-900 K, and stoichiometric ratios ranging from highly reducing to oxidizing conditions. The experimental results are interpreted in terms of a detailed kinetic model drawn from previous work of the authors, including an updated reaction subset for the direct interactions of NOx and C1-2 hydrocarbon species relevant to the investigated conditions. The results reveal a significant decrease in the initiation temperature upon addition of NOx. A similar effect is observed with increasing pressure. The sensitizing effect of NOx is related to the hydrocarbon chain-propagating NO/NO2 cycle operated by NO2 + CH3 reversible arrow NO + CH3O and NO + CH3OO reversible arrow NO2 + CH3O as well as the formation of chain-initiating OH radicals from interactions between NO/NO2 and the H/O radical pool. At low temperatures, reactions between NO/NO2 and CH3O/CH2O also gain importance. The results indicate a considerable intermediate formation of nitromethane (CH3NO2) as a characteristic high-pressure phenomenon. The formation of CH3NO2 represents an inactivation of NOx which may result in a temporary reduction of the overall hydrocarbon conversion rate. (C) 2008 The Combustion Institute. Published by Elsevier Inc. All rights reserved.