The structure, electronic conductivity and electrochemical properties of the oxide perovskites with the Ln(0.5)M(0.5)FeO(3-delta) composition (Ln = La, Sm; M = Ba, Sr) are studied in the context of their possible use as cathode materials for intermediate temperature solid oxide fuel cells (IT-SOFCs). The main objective of the work is to analyse the influence of the A-site cation size disorder (sigma(2)(r(A))) on the properties of these compounds. The A-site cation size disorder of these samples varies from 0.0286 to 0.089 angstrom(2), whilst the doping (x) and the average ionic radius of the A-site cations () are fixed to 0.5 and similar to 1.30 angstrom, respectively. The samples are prepared using the glycine-nitrate method. X-ray powder diffraction (XRD) shows that all of the compounds have rhombohedral symmetry (space group: R-3c). The structure is highly dependent on sigma(2)(r(A)); the lattice parameters and unit cell volume decrease with decreasing sigma(2)(r(A)). All of the samples have a porous microstructure with fine grain sizes (100-250 nm). The electrical conductivity (sigma) measurements on the bulk samples shows a strong dependence on the A-site cation size disorder. Electrochemical impedance spectroscopy (EIS) measurements of the cathodes on the yttria-stabilised zirconia (YSZ) electrolyte-based symmetrical cells indicate that the lowest polarisation resistance value is observed for the sample with the lowest sigma(2)(r(A)). The effect of introducing a samarium doped ceria (SDC) interlayer between the best performing cathode and the electrolyte is also studied. (C) 2013 Elsevier B.V. All rights reserved.