Si-rich phases and their distributions in the oxide scale formed on 304 stainless steel (304SS) kinetically affects SiO₂ volatilization for the formation of silicate-based Cs-bearing microparticles released during the Fukushima Daiichi Nuclear Power Plant accident. Therefore, we investigated Si-rich phases and their distributions in the oxide scale on 304SS oxidized in H₂O–H₂ mixtures over the temperature range 1000–1300 °C. SiO₂ film stays at the base of the Cr₂O₃-type scale formed at low temperatures or high H₂ contents in steam. Fe₂SiO₂ phase is non-uniformly distributed in the spinel-type scale formed under opposite conditions. Fe₂SiO₂ accumulation in the spinel-type scale is attributed to its thermodynamically metastable state in H₂O–H₂ mixtures. With increasing oxidation time, Fe₂SiO₂ phase was further oxidized into SiO₂ phase.