The Ba3InGa2O7.5 complex oxide, possessing the perovskite-related struc-Ture with structural oxygen vacancies, was first synthesized by the solid state method. The phase was found to be characterized by monoclinic sym-metry (sp. gr. P2/c) with the following unit cell parameters: A = 7.942(1) A, b = 5.868(5) A, c = 18.201(6) A, = 91.52(9). Comprehen-sive investigations of electrical properties were carried out; ceramic ma-Terial based on the complex oxide was shown to be a predominantly ionic conductor in the temperature range 450 900 oC. The conductivity is due to oxygen-ion transfer in dry conditions and oxygen-ion and proton transfer in wet atmosphere. The proton conductivity value is 4.5 10 5 S/cm, and the proton transport number is ∼50% at 700 oC in wet air; at lower tempera-Tures, proton transport becomes dominant. Prolonged treatment of the sam-ple in water vapors below 450 oC leads to hydrolysis decomposition.