This chapter talks about the defect properties of the AlNi alloy. Defect properties of the two long range ordering intermetallic compounds AlNi and AlNi3 have been investigated to some extent because of the possible technological application at high temperature. The annealing of AlNi3 after e--irradiation at 4.5 K is shown in the figure. Vacancy parameters have been measured by PAS and ΔQ after irradiation, quenching and cold work. The annealing of quenched in vacancies has been investigated by resistivity measurements for AlNi and void formation was observed by TEM during the annealing. The dependence of HF and of the vacancy concentration on the stoichiometry is shown in the figure. Isochronal annealing of polycrystalline AlNi3 after 3MeV electron irradiation at 5K and differential curves for three different alloy compositions after irradiation with the higher dose are illustrated. The measurements of the electrical resistivity were performed at 77 K and composition dependence of the apparent vacancy formation energy and activation energy for diffusion in NiAl are provided.