Mixer-settlers are widely employed for carrying out solvent extraction processes in various industries. In the mixer section, an emulsion of aqueous and organic phases is generated to achieve inter-phase mass transfer, which is followed by phase separation in the settler, where efficient separation is critical to the overall process economics. Settler performance depends on the flow pattern, which in turn is dependent upon various factors including settler internals, and inlet and outlet arrangements. A study was carried out to investigate the fluid flow behaviour inside an industrial settler, by means of both on-site measurements and computational fluid dynamics (CFD) modelling. An ultrasonic velocity profiler (UVP) was customized and used as the physical measurement tool. This instrument has the advantage of being able to make measurements in an opaque fluid. A highly non-uniform flow pattern was found, with regions of large-scale recirculation and short-circuiting. Comparison with the flow pattern obtained through CFD modelling demonstrated reasonable agreement. Since the flow pattern was considered to be poor, further numerical modelling was undertaken to investigate design improvements. Variations to several design parameters were assessed, mainly focussing on modification to the internal picket fences. Single-phase simulations were used as a fast screening tool, then the most promising modified designs were further assessed by two-phase simulations. Through quantitative assessment of the flow patterns obtained for proposed design modifications, the study has led to the identification of a design which substantially improves the flow pattern, which is expected to reduce entrainment losses and improve process economics. Crown Copyright (c) 2016 Published by Elsevier B.V. on behalf of The Institution of Chemical Engineers. All rights reserved.