Fatigue tests in reversed torsion were run on 2024-T351 aluminum alloy specimens into which hydrogen had been diffused. The diffusion was accomplished by placing the specimens in a hydrogen environment (>99.5%) for 25 days at 2000 psi [13.8 MPa] and 123°C. A control group was tested which underwent the same temperature conditions for 25 days. The fatigue tests were run at low (20-25%) and high (85-90%) relative humidities and at shear stress levels of approximately 13400, 16800, and 20100 psi [89.6, 117, and 138 MPa]. The results of this investigation show that hydrogen charging has no effect on the torsional fatigue life of aluminum. However, a change in the crack propagation angle at high relative humidity for a ductile, circumferential crack on uncharged specimens to a brittle, 45° crack on hydrogen-charged specimens may be the result of hydrogen embrittlement. / Master of Science