In order to explain the mass transfer enhancement factor obtained in slurry reactors, a static experimental system and a dynamic one are used to observe the behavior of fine activated carbon particles in the vicinity of a gas bubble. We first investigate the possible adhesion of fine particles in a static system. A gas bubble formed at a needle point, is maintained in a beaker filled with the liquid phase, while injections of concentrated activated carbon particles solutions are made with a syringe over the gas bubble. Visual observation of the bubble show that there is adhesion of particles on the gas bubble, which can be totally covered by activated carbon. This adhesion depends on the tonic strength and on the pH of the liquid solution. Further, we investigated the case of a static gas bubble in a hydraulic tunnel. Use of a movie camera allows td visualize the particles trajectories around the gas bubble. It has been found that there is no adhesion of the fine particles used in this study on the gas bubble when the liquid-solid solution is flowing, the particles follow the streamlines of the fluid; for the highest values of the fluid Velocity investigated, we observed a small helicoidal vortex of the carbon particles behind the gas bubble. No influence of the ionic strength, the pH nor of the particle diameter can be pointed out. The activated carbon particles behavior is obviously controlled by hydrodynamic forces in a dynamic liquid system. Observed phenomena are fundamentally different from the static case. Therefore, the enhancement factors obtained using these particles cannot be explained by particle adhesion at the interface.