An experiment is described which demonstrates the diffusion of interstitial impurities in alpha iron. It consists in the measurement of the logarithmic decrement of the oscillatory motion of a torsional pendulum utilizing a commercially available iron suspension wire of high purity. From the results obtained over a conveniently small temperature range, the activation energy for diffusion of the predominant impurity—nitrogen—can be found. This energy agrees favorably with earlier determinations made over a much wider temperature range using iron wire with specially introduced impurities.