Mid-infrared absorption spectroscopy has been applied to the detection of the hydroperoxyl (HO2) radical in pulsed laser,photolysis. combined with a laser, absorption kinetics reactor. Transitions of the nu(3) vibrational band assigned to the O-O stretch mode were probed. with a.. thermoelectrically cooled, continuous wave mid-infrared distributed feedback quantum cascade laser (QCL). The HO2 radicals were generated with the photolysis of Cl-2/CH3OH/O-2 mixtures at 355 nm. The absorption cross section at each pressure was determined by three methods at 1065.203 cm(-1) for the F-1, 13(1,13) <- 14(1,14) transition in the nu(3) band. From these values, the absolute absorption cross section at zero pressure was estimated. The relative line strengths of other absorptions in the feasible emitting frequency range of the QCL from 1061.17 to 1065.28 cm(-1) were also measured, and agreed with values reproduced from the HITRAN database. The nu(3) band absorption strength was, estimated from the analytically obtained absolute absorption cross section and the calculated relative intensity by spectrum simulation, to be 21.4 +/- 4.2 km mol(-1), which shows an agreement with results of quantum chemical calculations.