The coefficient of diffusion of hydrogen in ferritic iron was determined as a function of impurity content at room temperature. Oxygen in amounts of 150 to 400 ppm had no effect. Neither did nickel up to 0. 04%. with increase from 0. 04 to 0. 1% Ni the coefficient decreased by a factor of three. Investigations on 99. 9985% pure iron, electrolytic iron, and Armco iron indicate that the coefficient of diffusion of hydrogen decreases with increasing impurity content. For 0 to 0. 5% impurity an empirical equation is given.