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 O-3 + OClO instead of the commonly assumed ClO + O-2. The rate constants at high- and low-pressure limits were predicted to be k(2)(infinity) = 1.5 x 10(20)T(-1.1) exp(-18360/T) s(-1) and k(2)(0) = 3.76 x 10(25)T(-3.28) exp(-13890/T) cm(3) mol(-1) s(-1) in the temperature range 500-2500 K. For the bimolecular processes, the sum of the predicted abstraction (k(3)) 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 k(3) = 1.0 x 10(-16)T(1.44) exp(-469/T) cm(3) molecule(-1) s(-1). The abstraction rate constant for 500-2500 K can be expressed by k(3) = 8.69 x 10(-17)T(1.45) exp(-441/T) cm(3) molecule(-1) s(-1).