The rate constant for the self-reaction of the acetonyl peroxy radicals, CH3C(O)CH2O2, has been determined using laser photolysis/continuous wave cavity ring down spectroscopy (cw-CRDS). CH3C(O)CH2O2 radicals have been generated from the reaction of Cl atoms with CH3C(O)CH3, and the concentration time profiles of four radicals (HO2, CH3O2, CH3C(O)O2, and CH3C(O)CH2O2) have been determined by cw-CRDS in the near-infrared. The rate constant for the self-reaction was found to be k = (5.4 ± 1.4) × 10-12 cm3 s-1, in good agreement with a recently published value (Zuraski, K., et al. J. Phys. Chem. A 2020, 124, 8128); however, the branching ratio for the radical path was found to be ϕ1b = (0.6 ± 0.1), which is well above the recently published value (0.33 ± 0.13). The influence of a fast reaction of Cl atoms with the CH3C(O)CH2O2 radical became evident under some conditions; therefore, this reaction has been investigated in separate experiments. Through the simultaneous fitting of all four radical profiles to a complex mechanism, a very fast rate constant of k = (1.35 ± 0.8) × 10-10 cm3 s-1 was found, and experimental results could be reproduced only if Cl atoms would partially react through H-atom abstraction to form the Criegee intermediate with a branching fraction of ϕCriegee = (0.55 ± 0.1). Modeling the HO2 concentration-time profiles was possible only if a subsequent reaction of the Criegee intermediate with CH3C(O)CH3 was included in the mechanism leading to HO2 formation with a rate constant of k = (4.5 ± 2.0) × 10-14 cm3 s-1.