The indirect coal liquefaction route to produce transportation fuels, Le., the Fischer-Tropsch synthesis, may favorably be carried out in the slurry phase. The paper reports on measurements of gas holdup and heat and mass transfer in bubble column slurry reactors of 4.1 and 10 cm diameter. The study was done under conditions prevailing in the Fischer-Tropsch slurry reactor (Rheinpreusen-Koppers process). The results enable, at least, an estimation of the majority of hydrodynamic properties involved in designing a slurry-phase reactor. Under the conditions of the Fischer-Tropsch synthesis the slurry reactor can be treated as a two-phase system which permits some conclusions concerning the favorable flow regimes and gas velocities.