AbstractThe slow tensile straining of smooth specimens of mill annealed Ti6Al4V with different hydrogen contents revealed hydrogen induced slow strain rate embrittlement when the hydrogen content exceeded about 2000 ppm. The effect of hydrogen on the mechanical behaviour of the Ti6Al4V at lower hydrogen levels was not so pronounced and may be interpreted in terms of the partitioning of hydrogen between theαandβphases and the effect of hydrogen in solution on the lattice parameters of these phases. Hydrogen levels in excess of about 1200 ppm promote an increase in the amount ofβphase at the charging temperature employed and the effect of this on mechanical properties is discussed. The effect of hydrogen solubility and hydride precipitation at theα/βinterface on hydrogen induced slow strain rate embrittlement is also considered.MST/3431