The mechanical response of oxidized grain boundaries in a nickel alloy used for nuclear applications has been investigated by performing microcantilever bend tests. It was found that whilst failure can proceed along the oxide-metal interface not all oxidized grain boundaries exhibit intergranular failure. The presence of an external surface oxide has been identified as playing a crucial role in influencing the mechanical response. By removing the surface oxide, using a focused ion beam, tests were performed on the same grain boundaries with and without a surface oxide layer, and showed that surface oxides can suppress or delay fracture. Taking into account the effect of the surface oxide on microcantilever tests, it was possible to present the most accurate parameterization of the local stress at failure of oxidized grain boundaries to date and to predict the experimentally observed behavior via realistic cohesive damage finite element simulations, which further underline the experimental observations. (C) 2017 Elsevier B.V. All rights reserved.