The gas-phase chemistry of the hypergolic system CH3NHNH2 - monomethylhydrazine (MMH), with oxidizers NO2/N2O4 at room temperature and 1 atm N-2 was investigated experimentally using a gold-coated chamber reactor, coupled with a Fourier transform infrared (FOR) spectrometer. The IR-active species identified in the early reactions include HONO, monomethylhydrazinium nitrite (MMH-HONO), methyl diazene (CH3N=NH), methyl nitrate (CH3ONO2), methyl nitrite (CH3ONO), nitromethane (CH3NO2), methyl azide (CH3N3), H2O, N2O and NO. In order to elucidate the mechanisms by which these observed products are formed, we carried out quantum mechanics calculations [CCSD(T)/M06-2X] for the possible reaction pathways. Based on these studies, we propose that the oxidation of MMH in an atmosphere of NO2 occurs via two mechanisms: (1) sequential H-abstraction and HONO formation, and (2) reaction of MMH with asymmetric ONONO2, leading to formation of methyl nitrate. These mechanisms successfully explain all intermediates observed experimentally. We conclude that the formation of asymmetric ONONO2 is assisted by an aerosol formed by HONO and MMH that provides a large surface area for ONONO2 to condense, leading to the generation of methyl nitrate. Thus we propose that the overall pre-ignition process involves both gas-phase and aerosol-phase reactions. (C) 2013 The Combustion Institute. Published by Elsevier Inc. All rights reserved.