The self-reaction of ClO radicals has been investigated by ab initio molecular orbital and variational transition state theory calculations. Both singlet and triplet potential energy surfaces were predicted by the modified Gaussian-2 method. The reaction was shown to take place mainly over the singlet surface by forming ClOOCl (k(1)) and ClOClO (k(1)'). These association processes were found to be strongly pressure dependent and the predicted total rate constants are in good agreement with the experimental data. The predicted second- and third-order rate constants for formation of ClOOCl and ClOClO can be expressed, respectively, in units of cm(3) molecule(-1) s(-1) and cm(6) molecule(-2) s(-1) by k(1)(infinity)=1.6x10(-9)T(-0.67) exp(-64/T), k(1)('infinity)=6.4x10(-9)T(-0.78) exp(-76/T), and k(1)(0)=8.31x10(-20)T(-4.96) exp(-336/T), k(1)(0),=1.72x10(-14)T(-6.99) exp(-926/T) in the temperature range 180-500 K for N-2 as the third body. The observed T, P-dependent data could be best accounted for with the heat of formation of ClOOCl, Delta(f)H(0)(o)(ClOOCl)=29.4+/-1 kcal/mol. The formation of Cl-2+O-2 (2), Cl+ClOO (3), and Cl+OClO (4) products have been confirmed, with the predicted pressure-independent rate constants: k(2)=1.09x10(-13)T(0.66) exp(-1892/T); k(3)=1.36x10(-13)T(0.77)exp(-2168/T); k(4)=6.26x10(-11)T(0.005) exp(-2896/T), respectively, in units of cm(3) molecule(-1) s(-1), covering the temperature range 200-1500 K. These results are also in reasonable agreement with existing experimental kinetic data. (C) 2003 American Institute of Physics.