The gas-phase reaction of monomethylhydrazine (CH3NHNH2; MMH) with ozone was investigated in a flow tube at atmospheric pressure and a temperature of 295 ± 2 K using N2/O2 mixtures (3–30 vol% O2) as the carrier gas. Proton transfer reaction–mass spectrometry (PTR-MS) and long-path FT-IR spectroscopy served as the main analytical techniques. The kinetics of the title reaction was investigated with a relative rate technique yielding kMMH+O3 = (4.3 ± 1.0) × 10−15 cm3 molecule−1 s−1. Methyldiazene (CH3NNH; MeDia) has been identified as the main product in this reaction system as a result of PTR-MS analysis. The reactivity of MeDia toward ozone was estimated relative to the reaction of MMH with ozone resulting in kMeDia+O3 = (2.7 ± 1.6) × 10−15 cm3 molecule−1 s−1. OH radicals were followed indirectly by phenol formation from the reaction of OH radicals with benzene. Increasing OH radical yields with increasing MMH conversion have been observed pointing to the importance of secondary processes for OH radical generation. Generally, the detected OH radical yields were definitely smaller than thought so far. The results of this study do not support the mechanism of OH radical formation from the reaction of MMH with ozone as proposed in the literature.