The matrix isolation technique with Fourier transform infrared detection has been applied to determine the products of gaseous radical reactions. The gas phase reactions were carried out in a discharge flow system and about 1% of the gas mixture was deposited onto a low temperature target through a pinhole. A differential pumping scheme was employed to maintain the pressure of the cryosystem below 10−5 torr while that of the flow system was kept at about 2 torr. Species including HO2 (from the H+O2 reaction), ClO2 (from the Cl+O2 reaction) and ClO (from the Cl+O3 reaction) have been produced in the gas phase and were successfully trapped in matrices and detected with an FTIR spectrometer. In addition, both HCl and HOCl have been detected as the reaction products from the gaseous ClO+HO3 reaction. The production of HCl from the ClO+HO2 reaction may have a significant impact on catalytic ozone destruction in the atmosphere.