Oxygen production from BSCF (Ba0.5Sr0.5Co0.8Fe0.2O3-delta) and yttrium-doped BSCF (Ba0.5Sr0.5Co0.8Fe0.175Y0.025O3-delta) hollow fibres was investigated, and the role of yttrium in the crystal structure was further explored using high-temperature X-ray diffraction. Yttrium substitution acted to increase the oxygen flux significantly, from 4.9 to 7.0 ml cm(-2) min(-1) at 900 degrees C for the BSCF and the BSCFY membranes, respectively. Permeation was particularly enhanced at lower temperatures, between 66% and 92% over the range 650-800 degrees C. The lattice expansion determined from high temperature X-ray diffraction measurements in air was similar for both compositions, suggesting that the higher oxygen fluxes obtained for BSCFY hollow fibres could be attributed to the higher non-stoichiometry due to yttrium addition to the BSCF crystal structure. In addition, the improvement of oxygen fluxes for small wall thickness (similar to 0.3 mm) hollow fibres operating below the critical length (i.e. limited surface kinetics regime) indicates that yttrium has enhanced the surface exchange rates. XRD patterns showed split peaks around 2 theta 31 degrees and 56 degrees above 200 degrees C, likely corresponding to a coexisting hexagonal perovskite phase. This peak-splitting was more pronounced for BSCFY, suggesting that the kinetics of the hexagonal phase formation may be faster for the yttrium-doped perovskite. The lattice volume of BSCFY expanded more than BSCF when exposed to nitrogen at 900 degrees C, confirming a higher release of oxygen and enhanced oxygen non-stoichiometry. (C) 2012 Elsevier B.V. All rights reserved.