Arc magmas erupted at the Earth’s surface are commonly more oxidized than those produced at mid-ocean ridges. Possible explanations for this high oxidation state are that the transfer of fluids during the subduction process results in direct oxidation of the sub-arc mantle wedge, or that oxidation is caused by the effect of later crustal processes, including protracted fractionation and degassing of volatile-rich magmas. This study sets out to investigate the effect of disequilibrium crustal processes that may involve coupled changes in H₂O content and Fe oxidation state, by examining the degassing and hydration of sulphur-free rhyolites. We show that experimentally hydrated melts record strong increases in Fe³⁺/∑Fe with increasing H₂O concentration as a result of changes in water activity. This is relevant for the passage of H₂O-undersaturated melts from the deep crust towards shallow crustal storage regions, and raises the possibility that vertical variations in fO₂ might develop within arc crust. Conversely, degassing experiments produce an increase in Fe³⁺/∑Fe with decreasing H₂O concentration. In this case the oxidation is explained by loss of H₂ as well as H₂O into bubbles during decompression, consistent with thermodynamic modelling, and is relevant for magmas undergoing shallow degassing en route to the surface. We discuss these results in the context of the possible controls on fO₂ during the generation, storage and ascent of magmas in arc settings, in particular considering the timescales of equilibration relative to observation as this affects the quality of the petrological record of magmatic fO₂.