In order to obtain the diffusion coefficient of hydrogen in iron free from trapping by dislocations and impurities, electrochemical permeation measurements have been done using high purity iron (99.99 mass% Fe), pure iron (99.99 mass% Fe, high substitutional and low interstitial impurity contents), pure iron (99.94 mass% Fe), and commercially pure iron (99.83 mass% Fe) between 278 and 318 K. Though these specimens, in which the dislocation densities are (0.6–4.4)×10¹¹ m⁻², have been annealed at high temperatures (973–1223 K), the diffusion coefficients measured (D_a) are influenced by the trapping effect of dislocations. Therefore, the diffusion coefficients free from dislocation trapping (D_df), and free from dislocation and impurity trapping (D_H) have been calculated from D_a on the basis of the trapping theory. In the iron specimens of 99.99 mass% purity, the values of D_H are nearly equal to those of D_df. This indicates that D_a obtained in the 99.99 mass% purity iron specimens is little influenced by the impurity atoms. The temperature dependence of D_H has been determined as follows:
D_H(m²⁄s)=(5.8±0.1)×10⁻⁸exp(−(4.5±0.2)(kJ/mol)⁄RT).