Ignition delays have been investigated for mixtures of monomethylhydrazine (MMH) + O-2 + Ar at high temperatures (850-1440 K) in the pressure range 160-400 kPa, for equivalence ratios of 1-4.3 and in the Ar dilution range 82-96 mol%. The replacement of argon by nitrogen leads to a twofold increase of the delay. The addition of 0.88-2.4 mol% hydrogen to the MMH + O-2 + Ar mixture shows that hydrogen has, globally, a promoting effect on the ignition in MMH + O-2 + (Ar). Moreover, it is shown, with the help of a kinetic model that the addition of MMH by forming radicals during its oxidative decomposition promotes hydrogen oxidation in the temperature range 900-1000 K.