We report time-resolved measurements relevant to the atmospheric oxidation of tetrachloroethene (Cl2C= CCl2) and pentachloroethane (CCl3CCl2H). Cl atoms were produced by photolysis of Cl-2 at 351 nm, using the output from a pulsed excimer laser, in the presence of C2Cl4 and a large excess of O-2. Experiments were performed with and without NO present. The formation of the reaction products C(O)Cl-2 and CCl3C(O)Cl was followed, in real time, via absorption of infrared radiation provided by tunable diode lasers. Contrary to the finding of Hasson and Smith (J. Phys. Chem. A 1999, 103, 2031) but in agreement with the results of Thuner et al. (J. Phys. Chem. A 1999, 103, 8657), the relative yields of C(O)Cl-2 and CCl3C(O)Cl were found to be independent of the concentration Of C2Cl4. However, as found in both previous studies, the relative yields were sensitive to whether the CCl3CCl2O radical was formed by the reaction between CCl3CCl2O2 and NO or by the mutual reaction of two CCl3CCl2O2 radicals. From the results it was possible to estimate the branching ratio k(5a)/(k(5a) + k(5b7)) for decomposition of the CCl3CCl2O radical by the pathways CCl3CCl2O (+M) --> CCl3C(O)Cl + Cl (+M) and CCl3CCl2O (+M) --> CCl3 + C(O)Cl-2 (+M). With CCl3CCl2O formed from CCl3CCl2O2 + NO, (k(5a)/[k(5a) + k(5b)]) = 0.69 +/- 0.13, whereas with 2CCl(3)CCl(2)O formed from 2CCl(3)CCl(2)O(2), (k(5a)/[k(5a) + k(5b)]) = 0.91 +/- 0.21. Attempts to use literature values of the rate constants in modeling, (a) how the concentration of CCl3C(O)Cl varied with time following the initiation of reaction and (b) how the yield of CCl3C(O)Cl depended on the initial concentration of tetrachloroethene, were not successful. Agreement between the experimental results and the calculations could only be obtained using rate constants for the reactions between CCl3CCl2O2 and NO and between CCl3CCl2O2 and NO2 that were appreciably larger than previous values.