The mechanisms of formation of activated states upon diffusion of impurity interstitial atoms in metals have been identified, which allows formulation of the theoretical criterion for reliability of corresponding experimental data. This criterion is valid for almost all experimental results gained using precise techniques for bcc, fcc, and hcp metals. In case of hydrogen, oxygen, and nitrogen, most adequate are the models of potential diffusion barrier that are based on the elasticity theory. For carbon and boron, a less energy-taking mechanism is realized which involves the electron system rearrangement in a crystal during the diffusion jump. The accuracy of the theoretical criterion is in line with the capacities of modern precision techniques.