A Computational Fluid Dynamics (CFD) model is developed for describing the hydrodynamics of sieve trays. The gas and liquid phases are modelled in the Eulerian framework as two interpenetrating phases. The interphase momentum exchange (drag) coefficient is estimated using the Bennett et al. correlation as a basis. Several three-dimensional transient simulations were carried out for a rectangular tray (5 mm holes, 0.22 m x 0.39 m cross section) with varying superficial gas velocity, weir height and liquid weir lends. The simulations were carried out using a commercial code CFX 4.2 of AEA Technology, Harwell, UK and run on a Silicon Graphics Power Challenge workstation with six R10000 200 MHz processors used in parallel. The clear liquid height determined from these simulations is in reasonable agreement with experimental measurements carried out for air-water in a rectangular tray of the same dimensions. It is concluded that CFD can be a powerful tool for sieve tray design.