The Fe(II)- and alpha-ketoglutarate-dependent dioxygenases catalyze hydroxylation reactions of considerable biomedical and environmental significance. Recently, the first oxidized iron intermediate in the reaction of a member of this family, taurine:alpha-ketoglutarate dioxygenase (TauD), was detected and shown to be a high-spin, formally Fe(IV) complex. The demonstration in this study that decay of the Fe(IV) complex is approximately 30-fold slower when it is formed in the presence of 1-[2H]2-taurine provides evidence that the intermediate abstracts hydrogen from C1, the site of hydroxylation, and suggests that quantum-mechanical tunneling may contribute to C1-H cleavage.