Hydrogen-based direct reduction is a fossil-free method for ironmaking, which involves exposing solid iron oxides to gaseous hydrogen. The reduction kinetics is determined by solid-state diffusion and phase transformation, where particularly the phase transformation from Fe_1-xO (wüstite, x indicates the deficiency of Fe ions in the lattice) into alpha-iron is approximately an order of magnitude slower than the initial reduction of Fe₂O₃ (hematite) to Fe₃O₄ (magnetite). The reduction leads to mass loss (oxygen), which is accommodated by the formation of pores. Porosity profoundly influences the reduction, phase transformation, and oxygen removal kinetics. Yet, the underlying structural and chemical mechanisms in and at pores are unknown. This motivated us to conduct a coupled experimental and theoretical study on individual pores, using hydrogen-reduced single-crystalline wüstite as a model system. We reveal the three-dimensional (3D) morphology of non-percolating pores and their surrounding structure. The structure and chemistry of a single pore are used for the construction of a phase-field model to simulate the redox processes, considering the associated transport phenomena and phase transformation. We find that the redox product (water vapour) accumulates inside the pores and re-oxidizes the material that has already been fully transformed into alpha-iron. Our new insights into how this water confinement shifts the local equilibrium at the pore back towards wüstite, while the matrix remains alpha-iron, may explain the sluggish reduction of wüstite. Such redox-driven phase transformations, which proceed under non-canonical thermodynamic boundary conditions, are inherently associated with solid-state mass loss, vacancy, and pore formation.