The crystal chemistry and mixed conductor properties of the n = 2 member of the Ruddlesden-Popper (R-P) phases Sr3-xLaxFe2-yNiyO7-delta with 0 <= x <= 0.3 and 0 <= y <= 1.0 have been studied at high temperature. High-temperature X-ray diffraction and thermogravimetric measurements of the equilibrium pO(2) (10(-5)<= pO(2)<= 1 atm) in the temperature range 400 <= T <= 1000 degrees C indicate that the Sr3FeNiO7-delta phase is able to accommodate a large oxygen non-stoichiometry (delta similar to 1.5) without structural transformations. The electrical conductivity and oxygen permeability increase with the substitution of Ni for Fe in the range 550 <= T <= 1000 degrees C. The electrical transport of the Sr3FeNiO7-delta phase is thermally activated and the activation energy decreases with the substitution of Ni for Fe for a given oxygen content. The increase in the oxygen permeation flux with increasing Ni content is due to an increasing oxygen non-stoichiometry and a lower activation energy for permeation. (c) 2006 Elsevier B.V. All rights reserved.