Abstract—Fatigue thresholds and crack growth rates were studied in the experimental alloy, Ti—5Al—4Mo, as a function of temperature, grain size and hydrogen concentration. Deformation is confined to planar slip bands along which fracture occurred at low hydrogen concentrations. Hydrogen accelerated crack growth rates at various combinations of temperature, grain size and hydrogen with a corresponding change in fracture from “cleavage’ to interface phase fracture. In addition, hydrogen was found to promote interface phase formation. It is proposed that stress‐assisted hydrogen accumulation increases the interface phase hydrogen concentration which reduces the interface phase fracture stress. This process depends on the local stress andβ‐phase hydrogen concentrations, temperature and the time under load. At 340 K, hydrogen had a relatively small effect on crack growth due to a change in slip behavior with increased hydrogen