Ba(2)In(2(1-x))Ti(2x)O(5+x)square(1-x) (0 less than or equal to x less than or equal to 1) compounds have been prepared by solid state reaction. At room temperature (RT), when x increases, the progressive filling of oxygen vacancies, concomitant with the substitution of Ti for In, first induces (for 0 < x less than or equal to 0.075) a disorder in the plane of oxygen vacancies observed in Ba(2)In(2)O(5)square. Then, it suppresses the distortion to orthorhombic symmetry; for 0.075 < x less than or equal to 0.15, the symmetry becomes tetragonal and a formation of domains is observed. For 0. 15 < x < 1, all members adopt a disordered cubic perovskite (DCP) structure at RT. Conductivity measurements between 450 and 800 degreesC show that the change from brownmillerite to tetragonal structure when x increases from 0.075 to 0.1 induces a drastic decrease of the activation energy. The highest oxide-ion conductivity is observed for 0.1 less than or equal to x less than or equal to 0.33: similar to0.5 x 10(-2) S cm(-1) at 700 degreesC. (C) 2003 Elsevier B.V. All rights reserved.