The reaction of pentadienyl radicals (C5H7) with O-2 has been studied by a combination of pulsed laser photolysis and photoionization mass spectrometry, These radicals were generated either by the photolysis of 1,3-pentadiene or by a two step process, photolyzing carbon tetrachloride to form Cl atoms, which then abstracted a hydrogen atom from 1,4-pentadiene. The equilibrium between pentadienyl radicals, O-2 and pentadienylperoxy radicals could be observed over the temperature range 268-308 K. An analysis of the time-dependent signal of pentadienyl radicals was used to evaluate the equilibrium constant. From the temperature dependence of the equilibrium constant, the enthalpy change for the reaction C5H7 + O-2 --> C5H7O2 was found to be 56 +/- 5 kJ mol(-1). The C-O bond energy in the C5H7-O-2 adduct is weaker than those of allyl-type peroxy radicals. Possible values of the heats of formation of C5H7 and C5H7-O-2 radicals and the resonance stabilization energy of the C5H7 radical are discussed.