An investigation of the diffusion of electrolytic hydrogen through membranes of: (1) polycrystalline Armco iron; (2) single crystal Armcoiron; (3) zone refined iron; and (4) A.I.S.I. 4340 has been made. The temperature and stress dependence of the permeation rate through (1) and (4) was investigated, while (1) to (3) have been investigated under varying concentrations of hydrogen in the metal. From the results concerning Armco iron polycrystals and single crystals, and zone-refined iron it has been concluded that trace impurities and grain boundaries have negligible effects on the hydrogen permeation rate. Stress has been shown not to affect D, but it increases the solubility of hydrogen in the lattice. The D0 and ΔHp value for α-iron and the stress dependence of the hydrogen solubility indicate that the rate-determining step in diffusion is the formation of a cavity (a dilated octahedral hole) to accommodate the diffusion hydrogen. Maxima observed in the relation of the rate of permeation to time were explained in terms of ‘blister´ formation; the temperature dependence of the critical hydrogen concentration necessary to form these blisters is in concordance with this hypothesis. The nucleation sites for such blisters are aggregates of dislocations. The mechanism proposed to explain the maxima in the permeation transients was used to rationalize the existing discrepancies in diffusion data found in the literature. It is suggested that the crack initiation site for stress corrosion cracking may be a highly localized density of dislocations, on a metal surface, generated by blister formation due to hydrogen.