[en] The effects of electrochemically introduced hydrogen on the room temperature mechanical properties of two β titanium alloys, TIMETAL 15-3 (Ti-15V-3Cr-3AI-3Sn, wt%) and TIMETAL 21S (Ti-15Mo-3Nb-3Al, wt%) are compared. Solution annealed, peak aged (538 C, 8h), and duplex aged (440 C, 20h, 538 C, 1/2h) conditions are investigated. Bridgman notched tensile bars are employed to quantify the degree of embrittlement both by reduction in the maximum longitudinal stress developed at the centerline of the notch and the average effective plastic strain across the notch diameter at maximum load. Fracture paths are correlated with the slip behavior observed in solution annealed material. Possible hydriding of the α and β phases is investigated through x-ray diffraction. Results show that TIMETAL 21S is more susceptible to hydrogen embrittlement than TIMETAL 15-3 as evidenced by reductions in the longitudinal stress, plastic strain, and changes in fracture mode at hydrogen concentrations above 1,000 wt. ppm. Possible hydriding of a large volume fraction of the α or β phases was not observed over the range of hydrogen concentrations investigated. The increased susceptibility of TIMETAL 21S to hydrogen embrittlement is attributed to a high temperature, long time, solution treatment which removed heterogeneous nucleation sites from the grain interiors. Subsequent aging occurs preferentially on β grain boundaries and lastly in the grain interiors, resulting in fine intragranular precipitates. These fine α plates are readily sheared and promote planar slip. In contrast, a lower temperature, shorter duration solution treatment for TIMETAL 15-3, results in a material with more homogeneous, larger α precipitates, which, in turn, promote wavy slip. Results indicate that persistent planar slip exacerbates both hydrogen embrittlement and aqueous environmentally assisted cracking in metastable β titanium alloys