The mechanism of the reaction of hydroperoxyl radical with ozone has been studied as a function of temperature by measuring product branching ratios using isotopic labeling. In these studies (HO2)-O-18 was reacted with unenriched ozone in order to measure the relative importance of hydrogen abstraction and oxygen abstraction by ozone. In either case hydroxyl radical is a product. However, with isotopic labeling the two channels can be distinguished by detecting the OH product with an isotopically specific method : (HO2)-O-18 + O-3 --> (OH)-O-18 + O-2 + (OO)-O-18;(HO2)-O-18 + O-3--> OH + O-2 + O-18(2). We detect both (OH)-O-16 and (OH)-O-18 with high sensitivity and high specificity using laser-induced fluorescence in the 1-1 and 0-0 bands after pumping the P-1(2) transition of the 1-0, A-X band. The branching fractions were measured as a function of temperature between 226 and 355 K. At the highest temperatures studied, hydrogen abstraction by ozone accounts for 88% of the reactive encounters. This fraction increases with decreasing temperature to similar to 95% at 226 K. This temperature dependence is analyzed to find the difference in activation energy for the two channels. We find that the barrier to oxygen abstraction by ozone exceeds that for hydrogen abstraction by 1000 +/- 400 cal/mol. We conclude that hydrogen atom abstraction by ozone is the dominant pathway for this reaction at atmospherically relevant temperatures.