Crack velocities in heat treated AISI 4335V steel were measured as a function of H//2S additions to an H//2 gas environment. Tests were conducted at 22 to 27 degree C and 10 to 100 torr H//2 gas pressure. Data are presented showing the relationship between crack velocity and hydrogen pressure, at constant stress intensity. It is shown that crack velocities in the gas mixtures are three orders of magnitude higher than those predicted for the pure H//2 gas diluent. In addition, the results are shown to agree with those expected for a hydrogen permeation controlled crack growth mechanism.