The knowledge about the effects of solids on gas-liquid systems and the respective physical mechanisms are not yet totally clarified. In this work, the effect of the solids on the mass transfer characteristics in a bubble column was studied experimentally for the systems air/water/expandable polystyrene (EPS) beads and air/water/glass beads. Volumetric liquid side mass transfer coefficient, k(L)a, was determined under different solid concentrations (up to 30 vol.%), superficial gas velocities (up to 2.7 mm/s) and mean diameters (1100, 770 and 591 mu m for EPS and 9.6 mu m for glass beads). The presence of EPS solids affects negatively k(L)a being this effect more pronounced for the smaller particles. Also, a decrease in k(L)a occurs when the solid loading increases. Experiments done with large polystyrene particles (d(p) >= 591 mu m) contaminated with very fine EPS particles (d(p) congruent to 0.1 mu m) indicate that very fine particles play an important role on gas-liquid mass transfer. Mass transfer experiments in a hollow glass spheres three-phase slurry showed a dual effect of solids loading on k(L)a, contrarily to what happens with the previous particles. These results can be associated with the different surface properties of the particles studied. An empirical correlation for k(L)a on the experimental variables was developed. (c) 2011 Elsevier B.V. All rights reserved.