When solid-state redox-driven phase transformations are associated with mass loss, vacancies are produced that develop into pores. These pores can influence the kinetics of certain redox and phase transformation steps. We investigated the structural and chemical mechanisms in and at pores in a combined experimental-theoretical study, using the reduction of iron oxide by hydrogen as a model system. The redox product (water) accumulates inside the pores and shifts the local equilibrium at the already reduced material back toward reoxidation into cubic Fe_{1-x}O (where x refers to Fe deficiency, space group Fm3[over ¯]m). This effect helps us to understand the sluggish reduction of cubic Fe_{1-x}O by hydrogen, a key process for future sustainable steelmaking.