The design of the next generation of β-stabilized γ-TiAl based alloys as structural materials for high-temperature applications in aircraft engines requires the precise knowledge of the mobility of defects in the ordered βo phase. To reach this goal a Mo-rich prototype alloy has been specifically produced and investigated. The mobility of defects, between 600K and 1635K, has been studied by mechanical spectroscopy. The internal friction spectra show a relaxation peak P1 (at 1130K for 1Hz) superimposed to a high-temperature background. We demonstrate that the relaxation peak is taking place inside the βo phase and measure an activation energy of EP1 = 3.55 ± 0.05eV. An atomistic model is additionally proposed to explain this relaxation peak, which is attributed to a Zener-like mechanism of stress-induced Mo-Mo dipoles reorientation by exchange with a vacancy, and consequently the measured activation energy corresponds to the one for Mo diffusion in the βo phase.