Abstract The influence of oxygen on solidification behaviour of cast TiAl-based alloys containing 40–48 at.% of Al was studied. Twelve alloys with fixed Ti:Al ratios ranging from 1.08 to 1.5 and oxygen content of 0.1, 0.8 and 1.5 at.% were prepared by induction melting and casting in a cold crucible under protective atmosphere. Increasing oxygen content affects significantly the macrostructure of the as-cast ingots, increases volume fraction of the α phase formed during peritectic solidification and leads to a change of the β primary solidification phase to the α phase in ternary Ti–44.2Al–1.4O, Ti–47.3Al–0.9O and Ti–47.2Al–1.5O (at.%) alloys. In the case of the alloy with the α primary solidification phase, the partition coefficients achieve values of k Al / α s / l = 0.9 and k O / α s / l = 1.29 . In alloys containing 1.5 at.% of oxygen, it has been observed that the β/α solidification phase transition is shifted to a lower aluminium contents when compared to that of binary systems. Oxygen extends the stability of single α phase region.