In this paper we study the processes of O2 and H2O interaction with La0.9Sr0.1ScO3−δ. Using high temperature neutron powder diffraction (HT-NPD), the evolution of crystal structure and water uptake was investigated in the temperature range 25–700 °C at 21.3 kPa in O2 and O2 + D2O atmospheres. The results of HT-NPD show the influence of the water uptake by La0.9Sr0.1ScO3–δ bulk on the thermal expansion of the material. The kinetics of oxygen surface exchange was studied using pulsed isotope exchange technique. The measurements were carried out in the temperature range 300−900 °C at pO2 = 5.1, 12.2 and 21.3 kPa in dry and humid (pH2O = 2.6 kPa) conditions. For the first time the physical model describing the kinetics of oxygen surface exchange between the mixture of oxygen and water in the gas phase and a solid oxide has been suggested. The model includes following steps: oxygen dissociative adsorption, oxygen incorporation, hydrogen surface diffusion, water adsorption and dissociation. The rate-determining step of the exchange is considered. The influence of O2 humidification on the oxygen surface exchange is discussed.