A series of experiments was performed to investigate the heat transfer characteristics of a catalyst bed both during the Fischer–Tropsch Synthesis (FTS) reaction and when heated without reaction. These experiments were carried out in a tubular fixed bed reactor. Three materials, SiO2, TiO2, and SiC, which differ considerably in intrinsic thermal conductivity, were chosen to prepare supported cobalt catalysts. The heat transfer experiments were conducted under both reaction and non-reaction conditions: the former with FTS and the latter by means of a heater located at the center of the catalyst bed. Four groups of experiments were carried out for each catalyst bed: (a) before the catalyst was reduced; (b) after the catalyst was reduced; (c) while the FT reaction was taking place in the reactor; and (d) after the FT reaction. The effective thermal conductivity coefficients (keff) were calculated on the basis of the two-dimensional temperature profiles derived from the experiments. The substance of this paper is a detailed discussion of the effect of the liquid phase product on the keff, and the development of a correlation between the keff and the chain growth probability of the FT reaction. The keff under reaction conditions was found to increase by 2–3 fold when compared to the values in non-reaction cases.