During the initial phase of low-temperature Fischer-Tropsch synthesis, the pore system of the catalyst is filled with liquid hydrocarbons. The investigations in a magnetic suspension balance with a cobalt catalyst revealed that the filling period may take several days or even more and strongly depends on the conditions such as temperature and H-2-to-CO ratio. A numerical model of the pore filling process was developed to gain a better understanding of the parameters that determine this process and of their impact on the filling time. The model is in good agreement with the experimental data.