Materials of the K2NiF4 structure type, and in particular those with a stoichiometry of LaxSr2-xFeO4-delta, have been investigated as possible cathodes for solid oxide fuel cells. A range of compositions was studied and careful attention was paid to the oxygen stoichiometry. It was evident from the results that oxygen excess was incorporated into the lattice and that the DC conductivity was significantly influenced by oxygen content. Investigation of the materials at elevated temperature, indicated that at the operating temperatures of Solid Oxide Fuel Cells (SOFCs), these materials would remain stable with respect to temperature.