Abstract The migration barriers for self-interstitial defects in Fe in the presence of solute Cr has been investigated using density functional theory calculations. It is seen that the dumbbell migration barrier is lower for a mixed interstitial than for a pure Fe one, in agreement with experiments. As a consequence, single self-interstitials in dilute Fe–Cr alloys will associate themselves to solute Cr atoms, and under irradiation conditions, the Cr can diffuse via both interstitial and vacancy mechanisms. Furthermore, the stability of self-interstitial defects and defect clusters in pure Cr have been calculated, showing that they should be significantly less stable in Cr rich precipitates than in the matrix. The most stable self-interstitial in Cr is shown to be a low-symmetry 〈2 2 1〉 configuration.