The vacuum ultraviolet (VUV) photoionization of the methyl peroxy radical, CH3O2, and unimolecular dissociation of internal energy selected CH3O2+ cations were investigated in the 9.7–12.0 eV energy range by synchrotron-based double imaging photoelectron photoion coincidence (i2PEPICO). A microwave discharge flow tube was employed to produce CH3O2 via the reaction of methyl radicals (CH3) with oxygen gas. After identifying and separating the different sources of CH3+ from photoionization of CH3 or dissociative photoionization of CH3O2, the high resolution slow photoelectron spectrum(SPES) of CH3O2 was obtained exhibiting two broad bands superimposed with a complex vibrational structure. The first band of the SPES is attributed to the X3A′′ and a1A′overlapped electronic states of CH3O2+ and the second is assigned to the b1A′ electronic state with the help of theoretical calculations. The adiabatic ionization energy (AIE) of CH3O2 is derived as 10.215 ± 0.015 eV, in good agreement with high-accuracytheoretical data from the literature. The vertical ionization energy of the b1A' electronic state is measured to be 11.5 eV and this state fully dissociates into CH3+ and O2fragments. The 0 K adiabatic appearance energy (AE0K) of the CH3+ fragment ion is determined to be 11.1548 ± 0.020 eV.