Hydroxylation of aliphatic carbons by nonheme Fe(IV)-oxo (ferryl) complexes proceeds by hydrogen-atom (H center dot) transfer (HAT) to the ferryl and subsequent coupling between the carbon radical and Fe(III)-coordinated oxygen (termed rebound). Enzymes that use H center dot-abstracting ferryl complexes for other transformations must either suppress rebound or further process hydroxylated intermediates. For olefin-installing C-C desaturations, it has been proposed that a second HAT to the Fe(III)-OH complex from the carbon a to the radical preempts rebound. Deuterium (H-2) at the second site should slow this step, potentially making rebound competitive. Desaturations mediated by two related L-argininemodifying iron(II)- and 2-(oxo)glutarate-dependent (Fe/2OG) oxygenases behave oppositely in this key test, implicating different mechanisms. NapI, the L-Arg 4,5-desaturase from the naphthyridinomycin biosynthetic pathway, abstracts H center dot first from CS but hydroxylates this site (leading to guanidine release) to the same modest extent whether C4 harbors H-1 or H-2. By contrast, an unexpected 3,4-desaturation of L-homoarginine (L-hArg) by VioC, the L-Arg 3-hydroxylase from the viomycin biosynthetic pathway, is markedly disfavored relative to C4 hydroxylation when C3 (the second hydrogen donor) harbors H-2. Anchimeric assistance by N6 permits removal of the C4-H as a proton in the NapI reaction, but, with no such assistance possible in the VioC desaturation, a second HAT step (from C3) is required. The close proximity (<= 3.5 angstrom) of both L-hArg carbons to the oxygen ligand in an X-ray crystal structure of VioC harboring a vanadium-based ferryl mimic supports and rationalizes the sequential-HAT mechanism. The results suggest that, although the sequential-HAT mechanism is feasible, its geometric requirements may make competing hydroxylation unavoidable, thus explaining the presence of alpha-heteroatoms in nearly all native substrates for Fe/2OG desaturases.