An experimental setup for studying elementary reactions of FO radicals was described. Methods of generating FO radicals and the procedure for measuring their absolute concentrations were examined. The reaction: F + O3 --> FO + O2 was investigated in jet conditions with an ESR/IR LMR spectrometer. The value of k = (6.2 +/- 0.3).10(-12) cm3.sec-1 was obtained for the reaction rate constant at room temperature.