The photoionization of alkylperoxy radicals has been investigated using a newly developed experimental apparatus that combines the tunability of the vacuum ultraviolet radiation of the Advanced Light Source at Lawrence Berkeley National Laboratory with time-resolved mass spectrometry. Methylperoxy (CH3OO) and ethylperoxy (C2H5OO) radicals are produced by the reaction of pulsed, photolytically produced alkyl radicals with molecular oxygen, and the mass spectrum of the reacting mixture is monitored in time by using synchrotron-photoionization with a double-focusing mass spectrometer. The kinetics of product formation is used to confirm the origins and assignments of ionized species. The photoionization efficiency curve for CH3OO has been measured, and an adiabatic ionization energy of (10.33 +/- 0.05) eV was determined with the aid of Franck-Condon spectral simulations, including ionization to the lowest triplet and singlet cation states. Using the appearance energy of CH3+ from CH3OO, an enthalpy of formation for CH3OO of Delta H-f(0)o (CH3OO) = (22.4 +/- 5) kJ mol(-1) is derived. The enthalpy of formation of CH3OO+ is derived as Delta H-f(0)o = (1019 +/- 7) kJ mol(-1) and the CH3+-OO bond energy as D-0(o)(CH3+ -O-2) = (80 +/- 7) kJ mol(-1). The C2H5OO+ signal is not detectable; however, the time profile of the ethyl cation signal suggests its formation from dissociative ionization of C2H5OO. Electronic structure calculations suggest that hyperconjugation reduces the stability of the ethylperoxy cation, making the C2H5OO+ ground state only slightly bound with respect to the ground-state products, C2H5+ and O-2. The value of the measured appearance energy of C2H5+ is consistent with dissociative ionization of C2H5OO via the Franck-Condon favored ionization to the a (a) over tilde (1)A' state of C2H5OO+.