A mechanism is proposed to explain the change in the ductility of Ni-rich Ni3Al on addition of small amounts of boron. It is argued that grain boundary segregation of B is not the cause of the increase in ductility. Instead, Ni–B bonds and antisite Ni defects reduce the directionality of the bonding in ordered Ni3Al and enhance the ductility in the interior of grains so that deformation can be easily transmitted from one grain to another. Experimental evidence for this model is presented. An estimate of the concentration of B to achieve maximum ductility is made and the estimate agrees well with the experimental value.