Small Co clusters (d < 10 nm) supported over mixed La-Co-Fe perovskites were successfully synthesized. These catalysts were active for Fischer-Tropsch (FT). Depending on the Co to Fe ratios the mixed perovskite exhibited two different forms: the rhombohedral phase of LaCoO3 is maintained for the mixed perovskite when x > 0.5, the orthorhombic phase of LaFeO3 is found for x < 0.5. Interestingly only one of these structures is active for the FT reaction: the orthorhombic structure. This is most likely due to the capacity of this material to maintain its structure even with a high number of cation vacancies. These cations (mostly Co) were on purpose extracted and reduced. Magnetic measurements clearly showed their metallic nature. Rhombohedral Co-Fe mixed perovskites (x > 0.5) cannot be used as precursors for Fischer-Tropsch catalysts: their partial reduction only consists in a complete reduction of Co3+ into Co2+. The partial reduction of orthorhombic perovskites (x < 0.5) leads to active Fischer-Tropsch (FT) catalysts by formation of a metal phase well dispersed on a cation-deficient perovskite. The FT activity is related to the stability of the precursor perovskite. When initially calcined at 600degreesC, a maximum of 8.6 wt.% of Co-0 can be extracted from LaCo0.40Fe0.60O3 (compared to only 2 wt.% after calcination at 750 degreesC). The catalyst is then composed of Coo particles of 10 nm on a stable deficient perovskite LaCo0.053+Fe0.603+O2.40. Catalytic tests showed that up to 70% in the molar selectivity for hydrocarbons was obtained at 250 degreesC, 40% of which was composed of the C-2-C-4 fraction. (C) 2003 Elsevier B.V. All rights reserved.