Oxygen tracer self‐diffusion in LiNbO₃, LiTaO₃, and LiNb_0.15Ta_0.85O₃ single crystals between 880 and 1050 °C is investigated. ¹⁸O₂ isotope‐exchanged samples are analyzed by secondary ion mass spectrometry. The diffusivities of each of the three different materials can be described by the Arrhenius law with an activation enthalpy of diffusion of about 3.2–3.5 eV. The diffusivities are highest for LiNbO₃ and are lower by about one order of magnitude for LiNb_0.15Ta_0.85O₃ and LiTaO₃. The change of the pre‐exponential factor is identified as the reason for the difference in diffusivities. The experimental results are compared to defect formation energy calculations as given in literature and to energy barrier calculations for the diffusion of a single O vacancy as determined by nudged elastic band calculations based on density‐functional theory. An oxygen vacancy mechanism is suggested to govern diffusion. The difference in diffusivities is tentatively attributed to a different number of freely migrating vacancies, probably due to defect complex formation.