The products of the gas-phase reaction between HO2 and ClO have been measured by matrix-isolation/FTIR spectroscopy at a total pressure of 700 Torr and at 210, 240, 270, and 300 K. HOCl was detected as the dominant product, accounting for greater than or equal to 95% of reaction at all temperatures, showing that HO2 + ClO --> HOCl + O-2 is the major reaction pathway at high pressures and low temperatures. Evidence was found for ozone formation in this reaction at low temperatures, implying the presence of a second, minor (less than or equal to 5%) reaction channel : HO2 + ClO --> HCl + O-3. NO evidence for other product channels was obtained, precluding the formation of long-lived isomers of HClO3. The atmospheric implications of a second channel in which O-3 and HCl are products are discussed.