Hydrodynamics and gas-liquid mass transfer are studied in a bubble and slurry bubble column. The investigations are made in a small scale (ID 0.1 m; height : 2 m) column using two batches of polydispersed and dense pyrite particles (rho(s) = 4500 kg/m(3)). These experiments were also made with glass beads of different sizes (38, 85 and 160 mu m), to compare with literature results. The evolution of the gas holdup and the volumetric gas-liquid mass transfer coefficient with the particle diameter obtained in the case of glass beads indicates the existence of a critical particle diameter corresponding to the maximum values of these parameters.