The flow field of liquid in two dimensional and three dimensional models of horizontal primary oil/water separators has been studied experimentally using LDA and PDA measurement techniques. A CFD simulation has also been applied to these configurations to assess its suitability for further modelling work. It was found that the CFD results were in reasonable agreement with velocity measurements in a small two dimensional, rectangular model separator, but there were substantial differences between measured flow patterns and CFD predictions in a larger three dimensional cylindrical separator. Measurements were also made of particle size distribution at locations near the inlet of the small two dimensional model separator. These were found to be consistent with size dependent particle trajectories predicted by the CFD simulation. It was concluded that the CFD simulation may be used to investigate design developments of the small two dimensional separator but the simulation as used here is not suitable for use in the larger three dimensional separator. Further work is required to establish the cause of this limitation. It appears to be due to the limit on the number of computational cells permitted in the CFD simulation which in turn restricts the minimum volume of cells used in the CFD simulation.