This study focuses on the influence of particles concentration on the hydrodynamics of bubble column slurry reactors operating in the heterogeneous flow regime. Experiments were carried out in a 50 mm diameter glass column with paraffin oil as the liquid phase and glass beads of 40 mu m diameter as the solids phase. The particles concentrations studied were 5, 10 and 20% v. For interpretation of the experimental results a generalization of the ’two-phase’ model for gas-solid fluid beds was used to model the bubble hydrodynamics. The two phases are identified as follows : (i) a ’dilute’ phase consisting of the fast-rising ’large’ bubbles which traverse the column virtually in plug flow, and (ii) a ’dense’ phase which is identified with the liquid phase along with the solid particles and the entrained ’small’ bubbles. The ’dense’ phase suffers a considerable degree of backmixing. Dynamic gas disengagement experiments were carried out in the heterogeneous flow regime to determine the gas voidage in the ’dilute’ and ’dense’ phases. The experimental data show that increasing the solids concentration results in a pronounced decrease in the total gas hold-up but the influence on the ’dilute’ phase gas hold-up is negligible. The ’dense’ phase gas voidage suffers a significant decrease in gas hold-up due to enhanced coalescence of the ’small’ bubbles resulting from introduction of particles. The virtual independence of the ’dilute’ phase hold-up on the liquid phase properties was confirmed by measurements with ethanol, octanol, water and aqueous NaOH solutions. The ’dilute’ phase gas hold-up could be described using a bubble growth model which accounts for the influence of the column diameter and column height.