Hydrogen is an impurity that is often present in LiXO₃ (X= Nb, Ta) single crystals and related materials. In this context, the diffusion of hydrogen is an important process because it may influence the overall conductivity of the material. We investigated the diffusional hydrogen uptake in LiNb_0.15Ta_0.85O₃ single crystals at 600 °C. For the experiments, O₂ is bubbled through liquid deuterated water (D₂O), which leads to a saturation of the gas atmosphere with D₂O that is incorporated into the crystal during isothermal annealing. The diffusivities of deuterium during uptake were determined by infra-red spectroscopy. We identified a fast process that can be associated with tracer diffusion and a second slower process with an almost three times lower diffusivity.