Perovskites based on alkaline-earth containing lanthanum cobaltites exhibit high electronic and ionic conductivities but their application as cathodes for solid oxide fuel cells (SOFC) or gas separation membranes may be limited by thermal expansion and low stability of certain compositions. To identify the optimum composition regime, the following three series in the system La1-xSrxCo1-yFeyO3-delta were studied: (a) La1-xSrxCoO3-delta, (b) La0.3Sr1-yCo1-yFeyO3-delta and (c) La1-xSrxCo0.2Fe0.8O3-delta. All samples were investigated by four probe de conductivity measurements and dilatometry. For samples in group (a), La1-xSrxCoO3-delta, the conductivity at 800 degreesC increases with Sr content to a peak value at x = 0.3 and then decreases continuously. The thermal expansion coefficient (TEC) decreases first, then increases again. A minimum of TEC with alpha (30-1000 degreesC) = 18.5 X 10(-6) K-1 was obtained at x = 0.2. For samples of series (b), both the electrical conductivity and TEC decrease with iron content. Even though these materials have high electronic and ionic conductivities, it is difficult to utilize them as fuel cell cathode materials because their thermal expansion is not compatible with other cell components. These materials are, however, promising for gas separation membranes. (C) 2000 Elsevier Science B.V. All rights reserved.