aIn the literature, it is widely claimed that during the initial period of Fischer-Tropsch synthesis liquid higher hydrocarbons fill catalyst pores completely, at least for temperatures of less than 250°C at a typical pressure of about 2 MPa. This leads to diffusional restrictions in the porous network for particles with a size for industrial fixed-bed operation (> 1 mm), whereby catalyst effectiveness and product selectivity are strongly affected. However, under industryally relevant reaction conditions, our experimental and theoretical investigations on the interplay of reaction and diffusion in cobalt catalyst particles reveal that the pores are only partly filled with liquid higher hydrocarbons even at a very long time on stream of months or more for a chain growth probability below about 0.8. Experiments were conducted in a magnetic suspension balance using particles of technical size (dp = 3 mm), and a mathematical model was developed describing quite accurately the formation, vaporization and accumulation of liquid products and their C-number distribution in the porous particle.