Topochemical transformations occurring during the reduction of low-concentrated catalytic dispersions used for Fischer-Tropsch synthesis in a three-phase slurry reactor are investigated. As evidenced by dynamic light scattering and transmission electron microscopy, catalyst systems containing nanoparticles with sizes of 91 and 3 nm, respectively, are formed in systems containing cobalt at concentrations of 5 and 1wt %. After catalyst activation via the reduction of cobalt-containing particles by hydrogen, the size of the dispersed phase is 2-3 nm regardless of the content of cobalt in the suspension. The study of magnetic properties of suspension samples in situ indicates that metallic cobalt is formed during the process of catalyst activation, as confirmed by X-ray powder diffraction analysis.