The formation of HO2 in the reactions Of C2H5, n-C3H7, and i-C3H7 radicals with O-2 is investigated using the technique of laser photolysis/long-path frequency-modulation spectroscopy. The alkyl radicals are formed by 266 nm photolysis of alkyl iodides. The formation of HO2 from the subsequent reaction of the alkyl radicals with O-2 is followed by infrared frequency-modulation spectroscopy. The concentration of I atoms is simultaneously monitored by direct absorption of a second laser probe on the spin-orbit transition. The measured profiles are compared to a kinetic model taken from time-resolved master-equation results based on previously published ab initio characterizations of the relevant stationary points on the potential-energy surface. The ab initio energies are adjusted to produce agreement with the present experimental data and with available literature studies. The isomer specificity of the present results enables refinement of the model for i-C3H7 + O-2 and improved agreement with experimental measurements of HO2 production in propane oxidation.