Hydroperoxyl radical (HO₂) is produced from the reaction of hydroxyl radicals with organic compounds in the atmosphere, and by photolysis of formaldehyde. Detection of HO₂ is a challenge due to their low abundance and short lifetime. We have developed a flow injection method for HO₂ analysis that is based on the chemiluminescence produced when an aqueous solution of HO₂ and its conjugate base O₂^- react with MCLA, a synthetic analog of the luciferin from the crustacean Cypridina. The technique is linear at least up to 1-µM HO₂(aq), and has a minimum detection limit of 0.1 nM. We calibrate the instrument with stable aqueous O₂^- standards produced in submicromolar concentration using ⁶⁰Co gamma radiolysis. The method is simple, low-cost and has minimal size and power requirements, making it an attractive candidate for field deployment. We have demonstrated that the technique can be used to measure atmospheric HO₂ radicals following collection into aqueous solution in a glass coil scrubber.