The rate constants of the abstraction reaction Mu + HBr —• MuH + Br were measured using the well established μSR technique at TRIUMF. Measurements were made at various temperatures between 170 and 480 K, and fit to the Arrhenius equation both with and without an additional T½ dependence. The rate constants obtained were k(T) = (5.101 ± 0.106) x 10⁻¹¹ exp{-(0.560 ± 0.110) kcal mol⁻¹/RT} and k(T) = (0.183 ± 0.005) x 10⁻¹¹T½exp{-(0.286± 0.014) kcal mol⁻¹/RT} cm³ molecule⁻¹ s⁻¹. The low activation

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... nergy for such an exothermic reaction indicates that the barrier on the Mu + HBr surface is early. Kinetic isotope effects of the H/Mu + HBr reactions along with a lack of curvature in the Mu + HBr Arrhenius plot suggest that tunneling is relatively unimportant down to 170 K.