The potential-energy surface for the reaction of ClO with NO2 has been constructed at the CCSD(T)/6-311+G(3df)//B3LYP/6-311+G(3df) level of theory. Six ClNO3 isomers are located; these are ClONO2, pc-ClOONO, pt-ClOONO, OClNO2, pt-OClONO, pc-OClONO, with predicted energies relative to the reactants of −25.6, −0.5, 1.0, 1.9, 12.2 and 13.6 kcal mol−1, and heats of formation at 0 K of 7.8, 32.9, 34.4, 35.5, 45.6 and 47.0 kcal mol−1, respectively. Isomerizations among them are also discussed. The rate constants for the low-energy pathways have been computed by statistical theory calculations. For the association reaction producing exclusively ClONO2, the predicted low- and high-pressure-limit rate constants in N2 for the temperature range of 200–600 K can be represented by: (N2)=3.19×10−17 T−5.54 exp(−384 K/T) cm6 molecule−2 s−1and =3.33×10−7 T−1.48 exp(−18 K/T) cm3 molecule−1 s−1. The predicted low- and high-pressure-limit rate constants for the decomposition of ClONO2 in N2 at 200–600 K can be expressed, respectively, by =6.08×1013 T−6.54 exp(−13813 K/T) cm3 molecule−1 s−1and =4.59×1023 T−2.43 exp(−13437 K/T) s−1. The predicted values compare satisfactorily with available experimental data. The reverse Cl+NO3 reaction was found to be independent of the pressure, giving exclusively ClO+NO2; the predicted rate constant can be expressed as k(Cl+NO3)=1.19×10−9 T−0.60 exp(58 K/T) cm3 molecule−1 s−1.