The drift flux concept has been used to describe and analyse some hydrodynamic parameters such as the flow regimes and the retention phases in three-phase fluidised and fixed bed reactors. The effects of the gas, liquid and solid properties and the characteristics of the apparatus (distributor quality) on the hydrodynamics have been studied. Two hydrodynamic data banks as well as experimental results have been used to study the influence of the coalescence inhibiting behaviour of liquids and to deter-mine flow regime transitions and phase retentions. In heterogeneous regime, the ratio of the drift flux density to the superficial gas velocity tends to a limit, approximately independent on the liquid velocity in both fixed and fluidised beds. Two correlations of the drift flux have been developed by this analysis; the first of which is valid in the three-phase fluidised beds and the other in the three-phase fixed beds. (c) 2007 Elsevier Ltd. All rights reserved.