The unimolecular decomposition of symmetric ClO3 has been investigated at the G2M(CC2)//PW91PW91/ 6-311+G(3df) level of theory. The results Show that the main dissociation products are 3O + OClO instead of the commonly assumed ClO + O2. The rate constants at high- and low-pressure limits were predicted to be k2∞ = 1.5 × 1020T-1.1 exp(-18360/T) s-1 and k20 = 3.76 × 1025T-3.28 exp(-13890/T) cm3 mol-1 s-1 in the temperature range 500-2500 K. For the bimolecular processes, the sum of the predicted abstraction (k3) and association (k-2) values can reasonably explain the experimental nonzero intercepts of the strong pressure-dependent rate constants, with the predicted total rate constants agreeing closely with the experimental values. The association and abstraction rate constants in the temperature range 200-500 K can be represented respectively by k-2 = 40.1T-6.16 exp(-403/T) for 1 Torr He and k3 = 1.0 × 10-16T1.44 exp(-469/T) cm3 molecule-1 s-1. The abstraction rate constant for 500-2500 K can be expressed by k3 = 8.69 × 10-17T1.45 exp(-441/T) cm3 molecule-1 s-1.