AbstractAn analysis is given of the factors that determine the energy levels of impurity atoms segregated to grain boundaries, taking iron as the exemplary host metal. For an interstitial atom the energy level can be estimated by either the embedded atom method or the tight binding method. The first of these, which is appropriate for electron acceptor impurities, weakens the boundary and leads to intergranular brittleness. The second, which is appropriate for atoms which bond covalently, strengthens the boundary. The same factors apply to substitutional atoms, but an additional effect here is the loss of the coherency contributions of those host atoms which are replaced in the boundary by their impurity substitutes. The effects of various impurities in iron are discussed in terms of these three factors.MST/1189