The kinetics and mechanism of the CIO + CIO reaction and the thermal decomposition of ClOOCl were studied using the flash photolysis/long path ultraviolet absorption technique. Pressure and temperature dependences were determined for the rate coefficients for the bimolecular and termolecular reaction channels and for the thermal decomposition of ClOOCl. In order to determine channel-specific rate coefficients and to minimize complications associated with secondary chemistry, the reaction was studied over wide ranges of initial reactant stoichiometry and temperature. The rate coefficient for the termolecular association channel in the low-pressure limit, CIO + CIO (+M) --> ClOOCl (+M) (1), with N2 as a third body was measured over the temperature range 195-390 K and resulted in k1,N2(T) = (1.22 +/- 0.15) X 10(-33) exp{(833 +/- 34)/T} cm6 molecule-2 s-1 (+/- 2sigma error bounds). The 300 K rate coefficient for reaction 1 was measured for a number of bath gases. The results are k1,M (X 10(-32) CM6 molecule-2 S-1) = 0.99 +/- 0.05, 1.24 +/- 0.09, 1.71 +/- 0.06, 2.00 +/- 0.27, 2.60 +/- 0.17, 3.15 +/- 0.14, and 6.7 +/- 3.6 for He, O2, Ar, N2, CF4, SF6, and C12, respectively. The effective collision efficiency for M = C12 is very large and is likely due to a chaperone mechanism. Below 250 K, the reaction was in the falloff regime between second- and third-order kinetics. From the falloff data, the rate constant in the high-pressure limit, k(infinity)300, was estimated to be (6 +/- 2) X 10(-12) cm3 Molecule-I s-1. The Arrhenius expressions for the three bimolecular channels, ClO + ClO --> C12 + O2 (2), ClOO + Cl (3), and OClO + Cl (4), over the temperature range 260-390 K are k2(T) = (1.01 +/- 0.12) X 10(-12) exp{-(1590 +/- 100)/T} cm3 molecule-1 s-1, k3(T) = (2.98 +/- 0.68) X 10(-11) exp{-(2450 +/- 330)/71 CM3 molecule-1 s-1, and k4(T) = (3.50 +/- 0.31) X 10(-13) exp{-(1370 +/- 150)/71 CM3 molecule-1 s-1. These expressions lead to a value of (1.64 +/- 0.35) X 10(-14) cm3 molecule-1 s-I for the overall bimolecular rate constant (k2 + k3 + k4) at 298 K. The rate coefficient expression for ClOOCl thermal decomposition was determined to be k-1(T) = (9.81 +/- 1.32) X 10(-7) exp{-(7980 +/- 320)/T} cm3 molecule-1 s-1 over the range 260-310 K. From a Third Law analysis using equilibrium constants derived from measured values of k1 and k-1, the enthalpy of formation (DELTAH-degrees(f)(298)) of ClOOCl was determined to be 30.5 +/- 0.7 kcal mol-1. The equilibrium constant expression from this analysis is K(eq)(T) = (1.24 +/- 0.18) X 10(-27) exp{(8820 +/- 440)/T} cm3 molecule-1. From the observed activation energy for reaction 4 and the literature activation energy for reaction -4, the OClO enthalpy of formation was calculated to be 22.6 +/- 0.3 kcal mol-1.