The hydrodynamics of an up-flow three phase packed bed bioreactor were investigated experimentally using tracer techniques. The effects of the liquid and gas velocities, and the particle size on the Peclet number (Pe) and on the liquid hold-up (epsilon (L)) were determined. While the range of gas and liquid flow rate were chosen according to biotic conditions. It was found that the Pe was not influenced from liquid flow rate, however decreased with increasing gas flow rate and particle/reactor diameter ratio (d(p) / D-c). Also it was found that liquid hold-up was not influenced from gas flow rate, however increased with increasing liquid flow rate and decreased with increasing d(p) / D-c ratio. The correlation which connected the Bodenstein number to gas phase particle Reynolds number and d(p) / D-c ratio and the correlation which connected the liquid hold-up, liquid phase particle Reynolds number and d(p) / D-c ratio were found using Marquardt-Levenberg approach.