A prerequisite for environmental and toxicological applications of mercury (Hg) stable isotopes in wildlife and humans is quantifying the isotopic fractionation of biological reactions. Here, we measured stable Hg isotope values of relevant tissues of giant petrels (Macronectes spp.). Isotopic data were interpreted with published HR-XANES spectroscopic data that document a step-wise transformation of methylmercury (MeHg) to Hg-tetraselenolate (Hg(Sec)4) and mercury selenide (HgSe) (Sec = selenocysteine). By mathematical inversion of isotopic and spectroscopic data, identical δ 202 Hg values for MeHg (2.69 ± 0.04 ‰), Hg(Sec)4 (−1.37 ± 0.06 ‰), and HgSe (0.18 ± 0.02 ‰) were determined in 23 tissues of eight birds from the Kerguelen Islands and Adélie Land (Antarctica). Isotopic differences in δ 202 Hg between MeHg and Hg(Sec)4 (−4.1 ± 0.1 ‰) reflect mass-dependent fractionation from a kinetic isotope effect due to the MeHg → Hg(Sec)4 demethylation reaction. Surprisingly, Hg(Sec)4 and HgSe differed isotopically in δ 202 Hg (+1.6 ± 0.1 ‰) and mass-independent anomalies (i.e., changes in Δ 199 Hg of ≤0.3 ‰), consistent with equilibrium isotope effects of massdependent and nuclear volume fractionation from Hg(Sec)4 → HgSe biomineralization. The invariance of species-specific δ 202 Hg values across tissues and individual birds reflects the kinetic lability of Hgligand bonds and tissue-specific redistribution of MeHg and inorganic Hg, likely as Hg(Sec)4. These observations provide fundamental information necessary to improve the interpretation of stable Hg isotope data and provoke a revisitation of processes governing isotopic fractionation in biota and toxicological risk assessment in wildlife.