The effects of temperature and composition on hardness, electrical resistivity and thermoelectric power of the γ-phase were studied in relation to a covalent component of interatomic forces. From the temperature dependence of resistivity and thermoelectric power, it was possible to assume the existence of a narrow energy band of high density which seems to be related to a homopolar Al⁻-Al⁻ bond. The strength of the bond appears to be affected by an introduction of compositional defects. When the composition of the γ-phase deviates from stoichiometry, the strength would be reinforced by excess aluminum but weakend by excess magnesium. The temperature of hardness-softening is relatively high, 0.72–0.79T_m, suggesting the contribution of the homopolar bond to the mechanical properties of the compound.