The fluid dynamic characteristics of a structured bed type multiphase mini-scale reactor are investigated in this work. This type of reactor, which is called spiral reactor, consists of a small tube with an internal diameter a little larger than the catalyst extrudate diameter where the catalyst particles are introduced one by one forming a string bed in the spiral tube. Mock up tests were performed at ambient conditions using water and nitrogen as liquid and gaseous phase, respectively, in upflow and downflow operation mode. The effect of the gas and the liquid superficial velocities, the reactor internal diameter to catalyst diameter ratio and the length of the reactor on the axial dispersion, liquid hold-up and pressure drop were studied. The liquid dispersion is not extended and its impact on the spiral reactor performance is expected to be not significant. (C) 2007 Elsevier Ltd. All rights reserved.