We show experimental support for the predicted H/D transfer reactions in the product-bound state of BSS, since the enzyme catalyzes H/D exchange at the carboxymethyl side chain of the product benzylsuccinate (C3 atom). As expected, this reaction depends on the active, radicalcontaining state of BSS and even proceeds sequentially to exchange both protium atoms of C3 of benzylsuccinate. In addition, we also identified a D/H exchange activity during the BSS reaction with d8-toluene by confirming small amounts of d7-benzylsuccinate as a byproduct of the abundant d8-benzylsuccinate, which confirms H/D exchange at Cys493.