The cobalt-doped lanthanum-nickel oxide system, La4Ni((3-x))CoxO10 +/-delta (0.0 <= x <= 3.0, Delta x=0.2), was investigated as possible cathode materials for intermediate-temperature solid-oxide fuel cells. X-ray diffraction shows the presence of two structural phases in the series belonging to Bmab for 0.0 <= x <= 0.2, 0.8 <= x <= 2.0 and 2.6 <= x <= 3.0 and Fmmm for 0.4 <= x <= 0.6 and 2.2 <= x <= 2.4. All compositions are oxygen-deficient (delta < 0). Electrical conductivity measurements show a systematic decrease in the conductivity as cobalt content increases from x=0.0 to 2.0, and reverses for x > 2.0. AC impedance measurements of the x=0.4 composition in symmetrical cells with LSGM as an electrolyte show improved electrode performance over the parent nickelate La4Ni3O9.78. Long-term thermal stability studies show the x=0.4 composition to be more stable than the x=3.0 phase after annealing at 1173 K in air for 1 week making this material a viable candidate for cathodes in solid oxide fuel cells. (c) 2006 Elsevier B.V. All rights reserved.