The free energy simulation method and EAM type potentials are employed to study segregation to the Σ5 (310)/ tilt grain boundary and its free energy in ordered Ni3−xAl1+x alloys. For 300K≤T≤900K, there is weak Al and Ni segregation to the grain boundary in the stoichiometric and Ni-rich (76.6 at. %) alloys, respectively. In the Al-rich (73.5 at. %) alloy, however, there is strong Al segregation. Al segregation induces the formation of a thin, orderedx AlNi layer at the boundary. The width of this layer grows as the temperature is decreased to approximately 6 Å at 300K. The grain boundary segregation decreases the grain boundary free energy by less than 10% in the stoichiometric and Ni-rich alloys, and by 30% in the Al-rich alloys, respectively. These results suggest a new model to explain the observed brittleness of grain boundaries in polycrystalline Ni3Al. In this model, the brittleness of Ni3Al alloys is attributed to the formation of an intrinsically brittle, thin AlNi layer at grain boundaries. This brittle layer can be removed by increasing the grain boundary Ni content - either by changing the alloy composition or due to cosegregation with B.