The hypergolic interactions between monomethylhydrazine (MMH, CH(3)NHNH(2)) and various forms of nitric acid were studied by several different experimental techniques. High-speed videos were acquired to visualize the pre-ignition, ignition and post-ignition events when a fuel drop impinged on a pool of nitric acid. A three-stage process was identified in the temperature profiles obtained from drop tests using Al(2)O(3) coated fine-wire thermocouples placed in both the liquid-phase and gas-phase regions. The temperature rose rapidly from ambient levels to the boiling point in the first stage, from the boiling point to 280 degrees C relatively slowly in the second stage and from 280 degrees C to a flame temperature very rapidly in the third stage. The gaseous species evolved from reactions between MMH and nitric acid in these three stages were probed by a confined-interaction setup, in conjunction with rapid-scan Fourier transform infrared (FTIR) spectroscopy and time-of-flight mass spectrometry (ToFMS). The first stage involved liquid-phase reactions which formed the ionic compound monomethylhydrazinium nitrate (MMH center dot HNO(3)), as well as oxidation products methyl nitrate (CH(3)ONO(2)), methyl azide (CH(3)N(3)), N(2)O, H(2)O and N(2). The second stage involved the formation of an aerosol cloud which was mainly composed of monomethylhydrazinium nitrate. The third stage involved rapid gas-phase reactions leading to ignition. These third-stage reactions were initiated by the thermal decomposition of nitric acid, and the identified species in this stage include H(2)O, HONO, CH(3)ONO(2), CH(3)ONO, CH(3)N(3), CH(3)OH, CH(3)NH(2), CH(4), N(2)O, NO, N(2), and small amounts of HNCO, NH(3), HCN and CO(2). Some important pre-ignition reactions are proposed for both liquid and gas phases. (C) 2011 The Combustion Institute. Published by Elsevier Inc. All rights reserved.