The kinetics of the CH2CClCH2 + O-2 reversible arrow CH2CClCH2O2 (3,-3) reaction has been studied using laser photolysis/photoionization mass spectrometry. Room-temperature decay constants of the CH2CClCH2 radical were determined in time-resolved experiments as a function of bath gas density ([He] = (3-24) x 10(16) molecule cm(-3)). The rate constants are in the falloff region under the conditions of the experiments. Relaxation to equilibrium in the addition step of the reaction was monitored within the temperature range 340-410 K. Equilibrium constants were determined as a function of temperature and used to obtain the enthalpy of the addition step of reaction 3. At high temperature (730 K), no reaction of CH2CClCH2 with molecular oxygen could be observed and an upper limit to the rate constants was determined (3 x 10(-16) cm(3) molecule(-1) s(-1)). Molecular structures and vibrational frequencies of CH2CClCH2 and CH2CClCH2O2 were obtained in quantum chemical calculations and used to compute the reaction entropy. The experimentally determined equilibrium constants combined with the calculated reaction entropy resulted in an R-O-2 bond energy -DeltaH(298)degrees(3,-3) = 76.5 +/- 2.8 kJ mol(-1), which coincides with the R-O-2 bond energy for the allyl radical. The results demonstrate the absence of any effects of chlorine substitution in the beta position on the R-O-2 bond energy.