This study deals with the effects of electrolyte and particle concentrations on the gas holdup in both the homogeneous and the heterogeneous flow regimes in a slurry bubble column. Gas holdup measurements and video recordings of the bubble behavior were carried out in a 2D slurry column (0.015 x 0.30 x 2.00 m) under ambient conditions. The additions of electrolyte (sodium gluconate, 0.05-0.2 M) and of solid carbon particles (diameter 30 mum, 0.1-1.0 g L-1) both lead to a considerable increase in gas holdup. In both cases, critical concentrations exist above which no further increase in gas holdup is observed. The transition from the homogeneous to the heterogeneous regime is not significantly affected by electrolyte but is increased by the presence of particles. Three mechanisms are proposed that might account for the gas holdup increase resulting from particle and electrolyte addition. It is suggested that a layer of carbon particles around the gas bubbles results in a lower average bubble rise velocity. Both the addition of carbon particles and the addition of electrolyte lead to bubble stabilization, a decreased rate of coalescence, and thus a higher gas holdup. It is further suggested that the presence of electrolyte changes the surface tension, leading to smaller bubbles, a lower average bubble rise velocity, and thus a higher gas holdup. The combined addition of electrolyte and carbon particles confirms these hypotheses.