The oxide-ion conductivity and crystal structure of the perovskite-type (Ba1-x-ySrxLa5+y)(2)In2O5+y solid-solution system, which was derived from Ba2In2O5 with a brownmillerite-type structure, were investigated as a function of temperature and oxygen partial pressure. The oxide-ion conductivity was discussed from the viewpoint of the unit cell free volume. The oxide-ion transport number approached unity with increasing La content and the ion conductivity increased with the La or Sr content. The oxide-ion conductivity (1073 K) of (Ba0.3Sr0.2La0.5)(2)In2O5.5 exhibited a maximum value of 0.12 (S/cm), which exceeded that of yttria-stabilized zirconia. It was found that the oxide-ion conductivity of the present system was dominated by both the unit cell free volume and the amount of mobile oxide ion.