The mass transfer coefficients (k(L)a) and bubbles size distribution were measured in a large-scale (0.316-m inside diameter, 2.8-m high) slurry bubble column reactor (SBCR) for H-2, CO, N-2 and CH4 in two organic liquid mixtures (Isopar-M and a hexanes mixture) in the presence and absence of two solids (iron oxides catalyst and glass beads) in a wide range of pressure (0.17-0.8 MPa), superficial gas velocity (0.08-0.2 m/s), and solid concentration (0-36 vol%). The k(L)a values were found to intimately follow the behavior of the bubble size distribution under the operating conditions used, as they appeared to increase with pressure and gas velocity and dramatically decrease when operating with high solid concentrations. The effect of high solid concentrations in the reactor prevailed over the effect of pressure on the bubble size distribution by creating large gas bubbles with significantly small gas-liquid interfacial areas, leading to small k(L)a values. Thus, industrial SBCRs operating with high catalyst loading could likely operate in a mass transfer-controlled regime due to the expected low k(L)a values.