The hardness and defect morphology of alloys with the composition (Fe 5 5 - x Ni x )Al 4 5 , where 0.1 =< x =< 10 at.% following high-temperature heat treatment and subsequent low temperature annealing was studied. It was found that nickel additions significantly reduced the softening which occurs on low temperature annealing of FeAl and also significantly increased the hardness of this compound. Following low-temperature annealing <001> edge dislocations were observed to form. In addition cuboidal voids were formed in those alloys with less than 10 at.% nickel. It is believed that the hardening which occurs in these alloys is associated with the interaction of the nickel atoms with the thermal vacancies which form on high temperature heat treatment. Possible mechanisms for the hardening which occurs are discussed.