In view of the destructive ignitions that occur in hypergolic test firings of attitude-control motors under simulated high-altitude conditions, the preignition behavior of MMH, UDMH, A-50 and N2H4with N2O4was examined at low pressures and temperatures in the laboratory. At low NO2pressures and temperatures, such as those that would be encountered in combustion chambers during starts in space, the ignition of these fuels was significantly delayed. During the preignition period, relatively involatile reaction products were formed between NO2. and the fuels. Infrared spectrophotometry, mass spectrometry, differential thermal analysis, and other techniques were used to examine these preignition residues. The IR absorption curves were consistent with the view that the major component in the MMH residue that could behave as an explosive monopropellant was MMH.nitrate, IR spectra of the preignition residues formed with N2H4or A-50 suggest that hydrazine nitrate was the major constitutent of these residues. The preignition residues probably are a corrosion and a contamination threat to equipment on the exterior of flight vehicles.