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 (η phase) were observed. The novel phases are in equilibrium with the Ti2Ni-type solid solution phase (Ti1−xAlx)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 η phases.