Abstract: Apparatus and methods for testing the hydrogen-gas compatibilities, hydrogen-gas embrittlement susceptibilities, hydrogen-gas containment performances, and/or the hydrogen-gas pressure-cycling durabilities, of hollow enclosures (“test specimens”), with single-layer, double-layer, or multi-layer walls, composed of various barrier materials, are disclosed. Barrier materials include but are not limited to: carbon steel, stainless steel, copper, aluminum, a polymeric material (e.g., high-density polyethylene), and a liquid material (e.g., water, or an aqueous solution). The test gas is either high-purity hydrogen or a hydrogen-bearing gas mixture (e.g., hydrogen gas mixed with methane/natural gas and/or biomethane). A key piece of the testing equipment is an enclosure that surrounds the test specimen. Fabricated from high-strength, porous solid material (e.g.