<p>With the transition toward low-carbon steelmaking, the generation of electric arc furnace (EAF) slag will substantially increase in the coming decades. With up to 45 wt.% FeO, EAF slags represent a promising secondary iron resource. Recent studies have shown the potential of molecular hydrogen to reduce molten steelmaking slags; however, the fundamental thermodynamic behavior of liquid iron oxides under hydrogen remains poorly understood. We investigate, on a thermodynamics basis, the hydrogen-reduction of liquid EAF slag with 2.2 basicity (CaO/SiO₂) and varying FeO contents (9–39 wt. % FeO), and directly compare it with the conventional carbon-reduction through thermochemical simulations using equal mass-based reductant additions at 1750 °C. Hydrogen is predicted to achieve higher iron recovery yields than carbon in slags containing &gt; 24 wt. % FeO, primarily owing to the larger molar reductant input associated with the equal reductant mass addition adopted in the simulations. As the reduction progresses, and the oxygen activity in the slag decreases, the thermodynamic driving force for carbon reduction overcomes hydrogen’s molar advantage. Consequently, hydrogen milder reduction potential also lowers the coreduction extent of present impurity oxides compared with carbon, especially of P₂O₅. This might enable hydrogen to achieve higher-purity iron alloys than carbon while liquid iron oxides are abundant and the FeO activity remains high in the slag. Additionally, differences in oxide activities within the RO phase (FeO–MgO–MnO solid solution) explain why the RO phase evolves toward MgO-rich compositions during carbon reduction, whereas it remains a solid solution during hydrogen reduction. These findings provide fundamental thermodynamic insights into hydrogen-based reduction of liquid iron oxides and additional impurity oxides, and offer a basis for designing low-carbon strategies for EAF slag valorization.</p> Graphical Abstract <p></p>