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The dissociative photoionization processes of methyl hydroperoxide (CH 3 OOH) have been studied by imaging Photoelectron Photoion Coincidence (iPEPICO) spectroscopy experiments as well as quantum-chemical and statistical rate calculations. Energy selected CH 3 OOH + ions dissociate into CH 2 OOH + , HCO + , CH 3 + , and H 3 O + ions in the 11.4–14.0 eV photon energy range. The lowest-energy dissociation channel is the formation of the cation of the smallest “QOOH” radical, CH 2 OOH + . An extended RRKM model fitted to the experimental data yields a 0 K appearance energy of 11.647 ± 0.005 eV for the CH 2 OOH + ion, and a 74.2 ± 2.6 kJ mol –1 mixed experimental-theoretical 0 K heat of formation for the CH 2 OOH radical. The proton affinity of the Criegee intermediate, CH 2 OO, was also obtained from the heat of formation of CH 2 OOH + (792.8 ± 0.9 kJ mol –1 ) to be 847.7 ± 1.1 kJ mol –1 , reducing the uncertainty of the previously available computational value by a factor of 4. RRKM modeling of the complex web of possible rearrangement-dissociation processes were used to model the higher-energy fragmentation. Supported by Born–Oppenheimer molecular dynamics simulations, we found that the HCO + fragment ion is produced through a roaming transition state followed by a low barrier. H 3 O + is formed in a consecutive process from the CH 2 OOH + fragment ion, while direct C–O fission of the molecular ion leads to the methyl cation.