The direct gas phase oxidation of methane to formaldehyde has been investigated in the presence of NO2 at short residence time (20-80 ms). A quartz annular flow microreactor was used and the effects of various operating parameters (temperature, residence time, X-O2/X-CH4 ratio and NO2 concentration) were examined. Compared with the reaction without NO2, the addition of NO2 decreases the reaction initiation temperature by 300 K and it remarkably improves the CH4 conversion as well as the HCHO yield. High HCHO yields (>4%) were obtained at relatively high X-O2/X-CH4 ratio (3-7) and/or relatively high NO2 concentration (>0.1%). The best experimental yield is 9%, which was obtained at 24.3% of CH4 conversion. By comparison between both systems (CH4/O-2 and CH4/O-2/NO2), it was concluded that NO2 favors the oxidation pathway of methane, at the expense of the formation of C-2 hydrocarbons. Furthermore, much less CO2 was detected in the reaction with NO2 than that without NO2, which can be explained by the lower reaction temperature. (C) 2012 Elsevier B.V. All rights reserved.