An investigation has been made of the structure of certain alloys of the Ni-Al-Ta and Ni-Al-Hf systems. The compositions chosen for investigation were: Ni-28.75 at% Al-2.5 at% Ta; Ni-26.75 at% Al-2.5 at% Ta; Ni-20.0 at% Al-7.5 at% Ta; Ni-20.0 at% Al-7.5 at% Hf; Ni-22.5 at% Al-5.0 at% Hf. The Ni-Al-Ta alloys consisted of γ′ +β-phases (orβ-phase transformed during rapid cooling to a martensitic product). Two types of morphology were observed in the transformedβ-phase, one consisting of parallel sided plates and the other of acicular shaped plates; the plates contained fine twins on {1 0 1} b c t planes. The as-cast Ni-Al-Nf alloys consisted of the γ′-phase, together with eutectic regions ofβ-phase and (Ni, Al)7Hf2; both lamellar and discontinuous eutectic morphologies were observed. Extensive solid solution of tantalum and hafnium in the γ′-phase occurred.