Photolytic OH-initiated oxidation of cyclopentene, cyclohexene, and 1,4-cyclohexadiene have been investigated by using tunable synchrotron photoionization mass spectrometry. Electronic structure calculations (CBS-QB3) are employed in Franck-Condon (FC) spectral simulations of the photoionization efficiency curves (AIE) of the observed products. Cyclopentenol (cyclopenten-1-ol, 1-c-C5H7OH) and, its isomers cyclopenten-2-ol (2-c-C5H7OH) and cyclopentanone (c-C5H8=O), are detected from OH-initiated cyclopentene oxidation. The measured adiabatic ionization energy (AIE) of 1-C-C5H7OH is 8.4(+/- 0.1) eV, and that of 2-c-C5H7OH is 9.5(+/- 0.1) eV. The calculated AIE of possible cyclopentene oxidation products cis-1,2-epoxycyclopentane and 2,3-epoxycyclopentanol is 9.97 and 9.44 eV, respectively. Product spectra from OH-initiated oxidation of cyclohexene and cyclohexa-1,4-diene show a substantial contribution from linear aldehydes, indicating a prominent role for ring opening. Implications for the oxidation chemistry of cycloalkenes are briefly discussed.