Abstract The effect of mobile impurity doping on fracture behaviour has been investigated using plasma charging of light elements for Fe-2.5 wt.%Si alloy crystals with particular attention to the role of crack-impurity elastic interactions. FeSi crystals are markedly embrittled by plasma charging of helium as well as hydrogen at around room temperature, this being accompanied by slow crack growth. Neon charging contributes little to the embrittlement, but argon charging does not contribute. The crystals are also embrittled by nitrogen charging in the tests at 450 K and exhibit slow crack growth during the tests. Elastic analyses indicate that crack-impurity interactions are induced not only by the applied K I field but also by the stress modification due to ambient impurities in the presence of a crack. The interactions serve effectively to concentrate mobile impurities ahead of a crack tip, leading to the increase in the local stress intensity k I. The effect of interstitial impurities on crack extension is discussed in connection with the modification of stress states due to impurities around a crack tip.