The mechanisms for the reactions of ClO with C2H2 and C2H4 have been investigated at the CCSD(T)/CBS level of theory. The results show that in both systems, the interaction between the Cl atom of the ClO radical and the triple and double bonds of C2H 2 and C2H4 forms prereaction van der Waals complexes with the O-Cl bond pointing perpendicularly toward the π-bonds, both with 2.1 kcal/mol binding energies. The mechanism is similar to those of the HO-C2H2/C2H4 systems. The rate constants for the low energy channels have been predicted by statistical theory. For the reaction of ClO and C2H2, the main channels are the production of CH2CO + Cl (k1a) and CHCO + HCl (k 1b), with k1a = 1.19 × 10-15T 1.18 exp(-5814/T) and k1b = 6.94 × 10-21 × T2.60 exp(-6587/T) cm3 molecule-1 s-1. For the ClO + C2H4 reaction, the main pathway leads to C2H4O + Cl (k2a) with the predicted rate constant k2a = 2.13 × 10-17T 1.52 exp(-3849/T) in the temperature range of 300-3000 K. These rate constants are pressure-independent below 100 atm.