Changes of activity and selectivity during the initial phases of Fischer-Tropsch (FT) synthesis have been measured with three promoted cobalt catalysts. It is shown that the FT regime is formed in situ in a slow process lasting several days. A "construction" of the "true FT catalyst" is therefore assumed. Taking into account complementary investigations, this construction is assigned to the segregation of the catalyst surface caused by strong CO chemisorption. This process would be accompanied by an increase of the number of active sites and their disproportionation into sites of higher and lower coordinations, which would exhibit different catalytic properties. The observed initial activity and selectivity changes are well to be explained with this concept.