Horizontal separators use gravity settling to separate oil from water and other components of oil well riser streams. Inlet designs can induce non-uniform liquid flow across the separator＇s cross section with a substantial zone of re-circulation. To optimise oil/water separation, the Liquid should achieve near to plug flow, so that both phases experience a limited range of residence times. Internal fittings to improve flow uniformity have been investigated. A single perforated plate across the flow near the inlet increased uniformity of axial velocity across the separator cross section downstream of the baffle. A 10% free area baffle gave better uniformity of flow across the cross section compared to baffles having 5, 15 and 20% free areas. For a fixed free area fraction, the baffle hole size had only a small effect on the flow distribution even though the hole size was varied sufficiently to cover both laminar and turbulent flow through the holes. Two baffles gave further slight improvement in flow uniformity but only when the baffles were very closely spaced. Measurements on the flow of a 20% by volume oil in water mixture showed that the flow of this two phase mixture was similarly improved by the presence of a perforated baffle. (C) 2000 Elsevier Science S.A. All rights reserved.