The kinetics of the reactions CH3CCl2 + O-2 reversible arrow CH3CCl2O2 --> products (1) and (CH3)(2)CCl + O-2 reversible arrow (CH3)(2)CClO2 --> products (2) have been studied using laser photolysis/photoionization mass spectrometry. Decay constants of the radicals were determined in time-resolved experiments as a function of temperature (299-1000 K (reaction 1). and 299-700 K (reaction 2)) and bath gas density ([He] = (3-48) x 10(16) molecules cm(-3) (reaction 1) and (3-24) x 10(16) molecules cm(-3) (reaction 2)). At room temperature 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 ranges 430-500 K (reaction 1) and 490-550 K (reaction 2). Equilibrium constants were determined as functions of temperature and used to obtain the enthalpies of the addition step of the reactions 1 and 2. At high temperatures (600-700 K) the rate constant of reaction 2 is independent of both pressure and temperature within the uncertainty of the experimental data and equal to (1.72 +/- 0.24) x 10(-14) cm(3) molecule(-1) s(-1). The rate constant of reaction 1 is independent of pressure within the experimental range and increases with temperature in the high-temperature region : k(1)(791 K less than or equal to T less than or equal to 1000 K) = (1.74 +/- 0.36) x 10(-12) exp(-6110 +/- 179 K/T) cm(3) molecule(-1) s(-1). Structures, vibrational frequencies, and energies of several conformations of CH3CCl2O2, (CH3)(2)CCl, and (CH3)(2)CClO2 were calculated using ab initio UHF/6-31G** and MP2/6-31G** methods. The results were used to calculate the entropy changes of the addition reactions : Delta S-298(o) = -159.6 +/- 4.0 J mol(-1) K-1 (reaction 1) and Delta S-298(o) = -165.5 +/- 6.0 J mol(-1) K-1 (reaction 2). These entropy changes combined With the experimentally determined equilibrium constants resulted in the R-O-2 bond energies : Delta H-298(o) = 112.2 +/- 2.2 kJ mol(-1) (reaction 1) and Delta H-298(o) = 136.0 +/- 3.8 kJ mol(-1) (reaction 2).