New metal/oxide (Co-Fe) catalysts (with no reduction or thermal pre-treatment) are efficient to produce light hydrocarbons with a low selectivity in CO2 by the Fischer-Tropsch synthesis. The low selectivity in CO2 is due to the occurrence of the CO2/H-2 reaction. These materials are stable under reaction conditions, and only few carbides are formed during the Fischer-Tropsch reaction. X-ray analyses indicate that the most degraded phase is the (Co-Fe) alloy phase in CO/H-2 reaction and the spinel phase in the CO2/H-2 reaction. It was demonstrated that these composites do not behave as the simple sum of a spinel phase and a (Co-Fe) alloy but have their own properties. (C) 2000 Elsevier Science B.V. All rights reserved.