[en] The existing state of hydrogen isotopes in lithium oxide (Li₂O) was studied using Fourier transform infrared absorption spectroscopy (FT-IR). Infrared spectra in Li₂O single crystals which were treated by thermal absorption and consequent quenching were observed from room temperature to 773 K. Multiple peaks were observed in the O-D stretching vibration region and these peaks had different dependence on temperature. These peaks were attributed to the stretching vibrations of O-D presenting as LiOD or Li₂O-D⁺ in bulk Li₂O with or without defects. Above 623 K, most of the deuterium ions in Li₂O exist as Li₂O-D⁺ and the infrared absorption peak for them was observed around 2520 cm^-1. The absorbance decrease of this peak during the isothermal annealing was well analysed by a diffusion model. Diffusion behaviour of hydrogen isotopes in Li₂O was discussed from the in-situ observation of O-D