The diffusivity of protons in Sr2+-doped LaScO3 single crystals was determined by in-situ infrared absorption spectroscopy. It was found that the high-temperature absorption spectrum shows a broad peak due to O-H stretching bands, which differs from that observed at room temperature. The diffusion coefficient was evaluated by two different methods: (1) suddenly changing the water vapor partial pressure and (2) exchanging H2O and D2O-saturated atmospheres. The diffusion and the activation energy are limited mainly by the diffusivity of oxygen ion vacancies based on the first measurement method at high temperatures. The proton self-diffusion coefficients were obtained by using the second measurement method, and the activation energy is 0.51 eV. (C) 2007 Elsevier B.V. All rights reserved.