In order to study the formation of thermal vacancies in the Ti–Al alloy system, high‐temperature positron lifetime measurements together with a modeling of defect formation in the framework of nearest‐neighbor pair bonds were performed for α2Ti3Al and compared to recent results on γTiAl [U. Brossmann, R. Würschum, K. Badura, and H.‐E. Schaefer, Phys. Rev. B 49, 6457 (1994)]. Substantial increases of the positron lifetime τ were observed for Ti65.6Al34.4 and Ti77.1Al22.9 in the temperature range T≳1200 K where thermal vacancy concentrations above the detection limit of positron annihilation are expected from the model calculations for the α2 phase. Within the high‐temperature increase of the positron lifetime in the α2 and the β phase single‐component positron lifetime spectra were observed. This behavior is in contrast to the two‐component spectra observed conventionally at intermediate positron trapping rates and is attributed to a fast detrapping and retrapping of positrons at vacancies due to a low positron–vacancy binding energy. For this case, a vacancy formation enthalpy of HFV=(1.55±0.2) eV in α2Ti65.6Al34.4 and HFV=(1.8±0.2) eV in βTi77.1Al22.9 can be derived. These results are discussed in the context of recent 44Ti tracer diffusion studies. © 1996 American Institute of Physics.