The phase relations in the system of BaCoO3 (-) (delta)-BaInO2.5 in the temperature range 800-1150 degrees C were investigated and five phases with perovskite or its related structures were identified. In the range of 1050 to 1150 degrees C, there exists a wide solid solution BaInxCo1 - xOy(0.1 <= x <= 0.84, 2.54 <= y <= 2.59) with the cubic perovskite structure. At 1050 degrees C, the cubic phase transforms to an orthorhombic phase firstly at x = 0.55 and the cubic solid solution is separated into two regions, and narrowed with the decrease of the temperature. The structure of the cubic phase in In-rich region appears as a super-lattice (a') of the simple cubic phase (a(p)) with the modulation vector a'=2a(p). At 950 degrees C. a tetragonal phase BaIn0.64Co0.36Oy emerges and extends to a solid solution at lower temperature. The orthorhombic and tetragonal structures are studied by electron diffraction and powder X-ray Rietveld analysis. The size variance of B-site cations was applied to interpret the phase transition from the cubic to the orthorhombic at x = 0.55 for compounds BaInxCo1 - xOy. The cubic solid solution shows good conductivities at low temperature. (C) 2010 Elsevier B.V. All rights reserved.