Abstract: The steel industry’s transition to hydrogen-based ironmaking necessitates a deeper understanding of magnetite ore reduction, a crucial yet underexplored pathway for decarbonization. This study systematically investigates the combined effects of particle size and gangue composition on hydrogen-based reduction behavior of four industrial magnetite ore concentrates with varying CaO and MgO contents. Thermogravimetric analysis at 973 K, interrupted reduction experiments, and post-reduction characterization steps are used to evaluate reduction extent and phase transformations across different particle size fractions and bulk ores. The finer fractions generally exhibit faster and more complete reduction. However, this trend is overridden by gangue effects in certain ores. Magnetite ores with MgO as gangue tend to form magnesio-wustite solid solution (Mg,Fe)O during reduction, resulting in dense microstructures that impede hydrogen diffusion and limit reduction progress. In contrast, magnetite ores with CaO as gangue facilitate the formation of intermediate calcium ferrites, which promote porous morphology and enhanced reducibility. Notably, even the finer particles of ore containing MgO show a lower reduction degree than the coarser particles of the ore containing CaO as gangue. This highlights the dominant role of gangue composition in governing reduction kinetics, intermediate phase formation and final product morphology. These findings contribute to the growing knowledge necessary to enable fossil-free ironmaking by emphasizing the importance of considering both granulometric characteristics and heterogeneity when evaluating magnetite ores for hydrogen-based reduction.