Abstract A study of deuterium conductivity and diffusion in the oxide perovskite La 0.9Sr 0.1YO 3− δ is presented in this work. Deuterium ions were implanted into La 0.9Sr 0.1YO 3− δ (50 keV, 1×10 16 atoms/cm 2) and the corresponding deuterium depth profile was determined by SIMS and compared with a Monte Carlo simulation (TRIM96). This implant was used as a standard for the determination of deuterium concentration in a La 0.9Sr 0.1YO 3− δ sample pre-treated in D 2O atmosphere. In this way, it was fully confirmed that La 0.9Sr 0.1YO 3− δ incorporates water at high temperatures. The conductivity of La 0.9Sr 0.1YO 3− δ was measured in D 2O atmosphere and compared with other proton (deuteron) conductors. Concentration and conductivity data were used in conjunction to estimate the deuterium diffusivity and the constant of reaction of (heavy) water incorporation into LaYO 3. Some comments on the catalytic activity of this oxide are made.