On the basis of the information available, steels(ferritic, martensitic, and bainitic), nickel-base alloys, and titanium alloys become embrittled in pure-hydrogen-gas environments at ambient temperature. The embrittling effect is detected by making tension tests on sharp-notched specimens in an environment of high-purity hydrogen gas and, for comparison, tests on similar specimens in an inert gas at the same temperature and pressure. If the material is embrittled by hydrogen, its notch tensile strength will be reduced. The effect is more pronounced as the hydrogen-gas pressure is increased, but in some cases the embrittling effect has been observed at 1 atmosphere of pressure. The effect is more pronounced for the high-strength steels and high-strength nickel and titanium alloys than for the low-strength alloys. In unnotched specimens exposed to a pure-hydrogen environmental, hydrogen embrittlement manifiests itself as a decrease in ductility. Results of tests on stable austenitic stainless steels such as Types 310 and 316, or certain aluminum alloys such as 6061-T6, 2219-T6, and 7075-T73, and beryllium copper indicate that there is no significant evidence of embrittlement of these alloys in hydrogen gas at pressures up to 10,000psi.