A previous study of LaCoxFe(1-x)O3 perovskites demonstrated the potential Of Such materials to produce light olefins from syngas by Fischer-Tropsch synthesis. The partial reduction of LaCo0.4Fe0.6O3 generates small Co-0 particles in interaction with a deficient perovskite. However, the stability of the perovskite limits the amount of metal to 2% in weight. To increase the amount of active phase (metal) that can be extracted from the oxide precursor by reduction, we attempted to synthesize A-site deficient perovskites. La((1-y))Co0.4Fe0.6O3-delta series prepared by a sol-gel-like route revealed to be, instead of expected A-site deficient oxides, gamma-Fe2O3/LaCozFe(1-z)O3 composites. Magnetic nanocores of gamma-Fe2O3 are surrounded by a perovskite-type shell. The epitaxial growth of the perovskite phase on the cubic iron oxides drives its crystallization toward the same cubic system. The characterization of La(1-y)Co0.4Fe0.6O3-delta oxides are presented, and the structural hypotheses are discussed. The reducibility of such oxides is studied by thermoprogrammed reduction and Mossbauer experiments, and the formulae of the stable partially reduced materials are determined. These materials are composed of an equimolar Co-Fe alloy on a cubic perovskite enriched in iron with respect to that of the fresh catalyst. The gamma-Fe2O3 disappear; one part of the Fe3+ cations is reduced into Fe-0 in the alloy, and the other part of the Fe3+ "replaces" a part of the Co3+ cations of the perovskite, which also reduce into Coo in the alloy. After partial reduction, the reactivity of these oxides in Fischer-Tropsch synthesis is evaluated. Their efficiency to produce light olefins and their stability over 300 It of test is demonstrated. (c) 2005 Elsevier Inc. All rights reserved.