The rate constant and branching ratio for the reaction Cl+HO 2-HCl+O2 (1a) -OH+ClO, (1b) have been measured in a discharge-flow system with far infrared laser magnetic resonance detection of HO2, OH and ClO. Chlorine atom concentrations were determined by titration with O3 and detected as ClO. Measurements were made at low pressure (0.9-1.5 Torr) and at temperatures between 250 and 420 K under pseudo-first-order conditions. The total rate constant showed no significant temperature dependence k1 = (4.2±0.7)±10-11 cm-3 molecule-1 s-1. k1a = (1.8 ±0.5)×10-11 exp[(170±80)/T]cm3 molecule-1 s-1, and k1b = (4.1±0.8) ×10-11exp[(-450±60)/T] cm3 molecule -1 s-1, the latter corresponds to about 20% of k 1 at 298 K. The errors are 95% confidence limits and include a factor for possible systematic errors.