Results of measurements between 533 and 1093 °C of the diffusion coefficient of Fe59 in two single crystals of FeSi (7.64 and 11.1 at% Si, respectively) are presented. For the lower temperatures a newly developed chemical sectioning technique was employed. The results show a marked ferromagnetic effect, i.e. the activation energy is strongly peaked at the Curie temperature. The results are analysed in terms of the Zener‐LeClaire theory in combination with an argument that relates the enthalpy of vacancy formation with the change in magnetic specific heat of the matrix upon introduction of a vacancy. The latter argument can also be used exclusively when related to the introduction of an activated complex in the matrix.