The formation kinetics of an ultrafine Fischer-Tropsch catalyst based on a cobalt-polyvinyl alcohol (Co-PVA) composite has been studied using in situ magnetometry. The CoO to Co reduction process has been investigated. It has been established that the kinetic data on cobalt reduction at the step of thermolysis of the Co-PVA precursor are best approximated by the two-dimensional diffusion mechanism. The particle size of the metal phase of the composite has been measured by magnetic granulometry, and this method has been shown to be applicable to determination of size characteristics. It has been found that cobalt particles are partially encapsulated inside the pyrolyzed matrix; the proportion of encapsulated particles is about 54%.