The mechanism for the reaction Cl + ClOOCl has been investigated by ab initio molecular orbital and transition-state theory calculations. The result shows that the reaction can produce both Cl-2 + ClOO and Cl2O + ClO. The former product pair can be formed by direct and indirect abstraction paths. The direct abstraction path, which dominates at low temperatures, takes place barrierlessly with the rate constant k(1) = 1.53 x 10(-13)T(1.1) exp(118/T) cm(3) molecule(-1) s(-1) in the temperature range of 200-1000 K. The rate constant of the indirect path, which occurs via the two isomers of the Cl-O(Cl)OCl complex lying 3 kcal/mol below the reactants, k(1)' = 7.14 x 10(-12)T(0.86) exp(-2370/T) cm(3) molecule(-1) s(-1), accounting for less than 0.1% of the Cl-2 yield at 298 K but increasing to about 46% at 1000 K. The new Cl2O + ClO products can be produced by the fragmentation of the two complexes; the rate constant for their formation was found to be pressure-independent and can be expressed by k(2) = 2.19 x 10(-14)T(0.70) exp(-1110/T) cm(3) molecule(-1) s(-1) covering the temperature range of 200-1000 K. Above room temperature, Cl2O formation becomes competitive; for example, its branching ratios, k(2)/k(t), are predicted to be 0.19, 0.55, and 0.62 at 298, 500, and 1000 K, respectively. The predicted total rate constant agrees closely with available experimental results. The heat of formation of ClOOCl has been examined in detail using different reactions and computational methods. The results of these calculations converge to Delta(f)Hdegrees(0)(ClOOCl) = 30 +/- 2 kcal/mol.