This article reports on the influence of elevated pressure and catalyst particle lyophobicity at particle concentrations up to 3 vol % on the hydrodynamics and the gas-to-liquid mass transfer in a slurry bubble column. The study was done with demineralized water (aqueous phase) and Isopar-M oil (organic phase) slurries in a 0.15 m internal diameter bubble column operated at pressures ranging from 0.1 to 1.3 MPa. The overall gas hold-up, the flow regime transition point, the average large bubble diameter, and the centerline liquid velocity were measured along with the gas-liquid mass transfer coefficient. The gas hold-up and the flow regime transition point are not influenced by the presence of lyophilic particles. Lyophobic particles shift the regime transition to a higher gas velocity, and cause foam formation. Increasing operating pressure significantly increases the gas hold-up and the regime transition velocity, irrespective of the particle lyophobicity. The gas-liquid mass transfer coefficient is proportional to the gas hold-up for all investigated slurries and is not affected by the particle lyophobicity, the particle concentration, and the operating pressure. A correlation is presented to estimate the gas-liquid mass transfer coefficient as a function of the measured gas hold-up: k(1)a(1)/epsilon(g) = 3.0 root Du(b)/d(b)(3) s(-1) (C) 2009 American Institute of Chemical Engineers AIChE J, 56: 584-596, 2010