The development of intermetallic titanium aluminides has been driven by the aeronautic and aerospace industries because of the excellent mechanical properties and low density of γ-TiAl based alloys. Up to now, several generations of γ-TiAl based alloys were developed with increasing complexity of the alloy systems. Nb is one of the most important alloying elements in γ-TiAl alloys and although it is considered as a slow diffuser, its influence has not been fully quantified yet. In this work we demonstrate, through mechanical spectroscopy measurements conducted on several γ-TiAl based alloys with different Nb content, that Nb impedes the diffusion of Ti atoms in the α ₂-Ti ₃Al phase. Internal friction measurements show a relaxation peak P(α ₂), which is associated with short distance diffusion of Ti atoms in the α ₂ phase, involving stress-induced rotation of dipoles Al-V _Ti-Al, whose activation energy is dependent on the Nb content. The increase of the activation energy is quantified as ΔE _a(Ti)= 0.037 eV × at% Nb, being attributed to the next-neighbor interaction of Nb atoms with the local configuration of Ti-V _Ti. This mechanism also produces a further broadening of the relaxation peak, which is attributed to the near-next-neighbor interactions for high Nb contents. Finally, an atomic model for the mechanism responsible for this relaxation is proposed allowing to explain the observed experimental behavior.