FeO–Fe₂O₃–P₂O₅ and BaO–FeO–Fe₂O₃–P₂O₅ glasses were immersed in ultrapure water at 120℃ for 24–168 h. The weight loss of ternary barium iron phosphate glasses immersed for 168 h with a value of < 1.0 × 10⁻⁹ kg mm⁻² (< 0.03% of initial sample weight) was ten times smaller than that of the binary iron phosphate glass. A reaction layer was formed during immersion tests on the barium iron phosphate glasses. FE-SEM observations indicated the thickness of reaction layer increased with immersion time up to a thickness of 150–230 nm. Weight loss of dissolution samples was suppressed once the reaction layer reached a thickness greater than 150 nm. TEM-EDS analysis with selected-area electron diffraction patterns revealed the reaction layer consisted of 6-line ferrihydrite and non-crystalline phase containing phosphorus with a Fe–O richer and no Ba (< 1 at%) composition, compared to the original glass composition. The dissolution and precipitation process of ionic species with a role of Ba ions for local pH modification is discussed for the formation of the reaction layer.