An attempt has been made to evaluate the activation energy of interstitial diffusion in body‐centered cubic metals on the basis of the distortion energy necessary to open one of the flat interstitial cavities (½ 0 0) adjacent to an occupied one to a size equal to the diameter of the interstitial atom. The frequency on which the diffusion process depends, and hence the frequency value to be used in calculatingD0, is simply the maximum frequency of the lattice along the (100) direction. The results obtained forQandD0agree satisfactorily with the experimental data and thus show clearly the dependence of the activation energiesQon the elastic constants of the metal of the matrix and on the interstitial atom diameter. Moreover the theory confirms the origin of some observed relaxation effects still uncertain and the evaluation of the activation energies of interstitial atoms anchored around lattice defects such as dislocations.