Materials of the K2NiF4 structure type have been prepared and the electrical conductivity in air determined for a number of compositions in the LaxSr2-xFe1-yRuyO4+/-delta solid solution series including three with Ru substituted for Fe on the B site: La0.2Sr1.8Fe0.6Ru0.4O4+/-delta, La0.4Sr1.6Fe0.7Ru0.3O4+/-delta, and La0.6Sr1.4Fe0.8Ru0.2O4+/-delta. Overall the total conductivity values measured were lower than expected for the Ru-doped materials, with a peak conductivity of approximate to2 S cm(-1) at 700degreesC. In the undoped LaxSr2-xFeO4+/-delta materials, a significant jump in conductivity was observed between the x = 0.7 and 0.8 compositions and was related to the bonding in the materials and the Fe3+/Fe4+ redox behaviour. In all materials, the conduction behaviour was found to follow a semi-conducting trend. (C) 2004 Elsevier B.V. All rights reserved.