AbstractThe reactionhas been studied using a discharge‐flow tube with resonant fluorescence detection of HO. Measurements ofk1have been made at temperatures between 237 and 404 K. Our results and earlier work suggest that the rate constant has a minimum atT≃ 500 K. Below room temperature our results are given by the expressionk= (2.0 ± 0.4) × 10−14exp[(430 ± 60)/T] cm3molecule−1s−1, where the error limits include an estimate of the measurement accuracy. No dependence ofk1on pressure at 300 K is observed in studies between 1‐ and 10‐torr helium. An upper limit ofkb<1%k1a