Hydrogen isotope ratios in organic materials—like plant cellulose, sugars, or animal keratins—are powerful tracers of water, climate, and metabolism. But part of the hydrogen in these molecules can exchange with ambient water, which means labs must “equilibrate” samples before measuring the hydrogen isotope composition. In our new study from PPE group members Dan Nelson, Ansgar Kahmen, Jochem Baan, Selina Hugger, Cristina Mercoli, and Meisha Holloway‐Phillips, we directly compared two established dual-water equilibration approaches: hot vacuum autosampler equilibration versus hot atmospheric-pressure steam equilibration. We showed that atmospheric-pressure methods generally drove more exchange (not just drying), and their measured hydrogen isotope ratios agreed better with independent chemical-derivatization results, while the vacuum autosampler results aligned better with some room-temperature-equilibrated reference materials. The upshot: different methods reproducibly access different pools of “exchangeable” hydrogen—some of which are hard to liberate from interior hydrogen bonds in complex structures like cellulose or keratin—so method choice should match the application, and methods should be reported clearly for cross-lab comparison. Read the paper in Rapid Communications in Mass Spectrometry.