The mechanism of the hydrazine-nitrogen tetroxide reaction was investigated by examination of the variation in reaction products and equilibrium reaction temperature with increasing reactant concentration when argon-diluted reactant streams were rapidly mixed in a flow reactor. It is concluded that the reaction proceeds through a thermal reaction mechanism rather than a chain-branched explosion mechanism. An ignition test was devised to determine the effect of candidate reaction inhibitors. Thiiophene, toluene, trimethyl borate, ethyl bromide and benzene produced some inhibition, but only at high concentrations. These results support the conclusions that a thermal mechanism is operative.