The effect of a range of preparation variables such as the precipitate ageing time and [Fe]/[Co] molar ratio of precipitation solution on the composition and morphology of iron-cobalt oxide catalysts prepared using a co-precipitation method is described and the optimum preparation conditions were identified with respect to the catalyst activity for the Fisher-Tropsch reaction. The effect of different promoters along with loadings of optimum support and promoter on the activity and selectivity of the 40%Fe/60%Co as an optimum molar ratio are studied and it was found that the catalyst containing 40%Fe/60%Co/15 Wt%SiO2/1.5 wt%K which aged for 2 h, is an optimum modified catalyst for the conversion of synthesis gas to ethylene and propylene. The results are interpreted in terms of the structure of the active catalyst. Characterization of both precursors and calcined catalysts were carried out using XRD, SEM, BET specific surface area and thermal analysis methods such as TGA and DSC. It was shown that all the different preparation variables influenced the catalyst precursor structure and morphology. (c) 2005 Elsevier B.V. All rights reserved.