The experimental evaluation of phase equilibria in the Ti-Ni-Al-N and Ti-Ni-Al-O phase diagrams are based on alloy samples, which were prepared of elemental powder blends by argon-levitation melting in a Hukin crucible. The experimental investigation employed X-ray powder diffraction, metallography, SEM, and EMPA techniques in the as-cast state as well as after annealing at 900°C. Two quaternary compounds Ti3NiAl2N and Ti3NiAl2O deriving from the filled Ti2Ni type (? phase) were observed. The novel phases are in equilibrium with the Ti2Ni-type solid solution phase (Ti(1-x)Al(x))2Ni, which exhibits a maximum solubility of 14 at % Al in binary Ti2Ni. Atom order in all these phases was monitored by quantitative X-ray powder diffraction (Rietveld analyses). The difference of X-ray spectra among the various phases deriving from parent Ti2Ni type was analyzed and the complex atom site occupation mode was discussed in terms of the general classification scheme for ? phases. (C) 2000 Academic Press.