The current work is focused on the impact of the lithium stoichiometry on electrical conductivity, acoustic properties, and high‐temperature stability of single crystalline Li(Nb,Ta)O₃ at high temperatures. The crystals grown from Li‐deficient melts are treated by the vapor transport equilibration (VTE) method, achieving near stoichiometric Li‐content. It is shown that the VTE‐treated specimens generally exhibit lower conductivity at temperatures below 800 °C, which is attributed to the reduced number of Li‐vacancies in near stoichiometric Li(Nb,Ta)O₃, provided that the Li‐ion migration dominates the conductivity in this temperature range. Further, it is shown that above 600–650 °C different mechanism increasingly contributes to the conductivity, which is consequently attributed to the electronic conduction. Further, it is shown that losses in LNT strongly increase above about 500 °C, which is interpreted to originate from conductivity‐related relaxation mechanism. Finally, the thermal stability of Li(Nb,Ta)O₃ is evaluated by the measurement of the conductivity and resonance frequency as a function of time. It is found that during annealing at 700 °C for 350 h, the resonance frequency of LiNbO₃ remains in a ± 100 ppm range of the initial value of 3.5 MHz.