The effect of lowering the A-site stoichiometry of La-Fe-Ni based perovskite solid oxide fuel cell cathodes was investigated with electrochemical impedance spectroscopy on cone-shaped electrodes using a Ce0.9Gd0.1O1.95 electrolyte. It was shown that a lowering of the A-site stoichiometry lowers the amount of Ni in the perovskite phase, as powder XRD revealed that NiO was expelled from the perovskite lattice when the A-site stoichiometry was decreased. NiO inhibit the reduction of oxygen as the activity of a nominally A-site deficient La1-sFe0.4Ni0.6O3-delta perovskite was worse than the activity of the corresponding LaFe0.4+sNi0.6-sO3-delta perovskite without NiO. NiO is therefore poison for the reduction of oxygen at the cathode in a solid oxide fuel cell. (C) 2009 Elsevier Ltd. All rights reserved.