Hydrogen embrittlement of 26Cr-lMo ferritic stainless steels, with low and high concentrations of interstitial elements of carbon and nitrogen and with high-temperature an nealing or prestraining treatments or both, was investigated. Tests involved cathodic charging of the specimens in sulfuric acid solution at room temperature, with simultaneous tensile loading using a uniaxial constant load fixture. The steel with high interstitial contents (26-1S) hydrogen embrittled intergranularly, when either heated to 1050°C and subsequently water quenched, or plastically prestrained by 5% elongation; but the low interstitial alloy (E-Brite) hydrogen embrittled transgranularly only when both of these treatments were given in this order. The cracks originated at the surface grain boundaries in 26-1S and at interior precipitate regions in E-Brite. Based on interrupted tests and fractography, the inferred hydrogen embrittlement mechanism has been stress-induced niobium hydride formation in E-Brite; whereas, this mechanism has been hydrogen trapping and absorption by nitrogen and faceted titanium carbo-nitrides in the vicinity of grain boundaries in 26-1S. Stress corrosion crack propagation of these alloys in boiling chloride solutions can be analyzed from these mechanisms and invoking potential drop concepts.