A process for the preparation of a packed bed comprising an iron enriched cobalt catalyst for use in a Fischer-Tropsch reaction, the process comprising the steps of: (a) providing a packed bed with one or more catalyst particles comprising metallic cobalt; (b) contacting a part of the catalyst particle(s) in the packed bed with an iron containing compound. The process is preferably conducted in situ which conveniently results in an iron containing cobalt catalyst with a higher C5+ selectivity. In certain preferred embodiments the concentration of iron increases towards the surface of the resulting catalyst particles whereas the cobalt concentration is constant which further increases the selectivity of the catalyst to producing C5+ hydrocarbons.