This work investigates the partial substitution of yttrium in place of iron in BSCF to form Ba(0.5)Sr(0.5)Co(0.8)Fe(0.2-x)Y(x)O(3-delta), where x varied between 0 and 0.2. X-ray diffraction patterns showed the formation of a perovskite cubic phase structure up to x = 0.15, whilst the full substitution of yttrium (x = 0.2) for iron resulted in a non-cubic crystal structure that did not exhibit oxygen permeation. The yttrium partial substitution in BSCFY discs (1.2 mm thick) delivered best results for x = 0.025, as oxygen fluxes reached 2.05 ml cm(-2) min(-1) at 900 degrees C, an increase of 160% as compared to a blank BSCF (x = 0) membrane. This was attributed to the combined effect of the formation of a greater number of oxygen vacancies, together with improved ion mobility, associated with the beneficial yttrium substitution into the BSCF perovskite structure which stemmed from the crystal lattice expansion. (C) 2011 Elsevier B.V. All rights reserved.