The decomposition of hydrazine, hydrazine-water mixtures, UDMH, and monomethylhydrazine were studied in the Princeton adiabatic flow reactor. The overall reaction order of all three monopropellants was found to be very close to unity. A comparison of the reaction rates showed that, in the temperature regime of this study, UDMH decomposition is fastest, hydrazine decomposition is slowest, and the monomethylhydrazine decomposition rate is intermediate. Reaction mechanisms for the thermal gas phase decomposition of hydrazine, and its methyl derivatives, were postulated and studied numerically. A study of hydrazine-water mixtures showed their rates to be slower than those of the anhydrous material by approximately a factor of 10, and independent of the amount of water added. Slightly wet hydrazine behaved kinetically like the hydrazine-water mixtures.