The gas-phase chemistry of the hypergolic system CH_3NHNH_2 – monomethylhydrazine (MMH), with oxidizers NO_2/N_2O_4 at room temperature and 1 atm N_2 was investigated experimentally using a gold-coated chamber reactor, coupled with a Fourier transform infrared (FTIR) spectrometer. The IR-active species identified in the early reactions include HONO, monomethylhydrazinium nitrite (MMH·HONO), methyl diazene (CH_3N=NH), methyl nitrate (CH_3ONO_2), methyl nitrite (CH_3ONO), nitromethane (CH_3NO_2), methyl azide (CH_3N_3), H_2O, N_2O 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 NO_2 occurs via two mechanisms: (1) sequential H-abstraction and HONO formation, and (2) reaction of MMH with asymmetric ONONO_2, leading to formation of methyl nitrate. These mechanisms successfully explain all intermediates observed experimentally. We conclude that the formation of asymmetric ONONO_2 is assisted by an aerosol formed by HONO and MMH that provides a large surface area for ONONO_2 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.