Mixing efficiency of three Hartridge-Roughton mixers with different geometries is characterized by a segregation index measured via the Villermaux/Dushman test reaction and by Computational Fluid Dynamics. The small values of the segregation index (always lower than 0.025 for the concentrations and the flow rate studied) confirm these vortex mixers achieve very good mixing performance. The use of the Turbulent Mixing Model indicates that mixing times are less than 5 ms for inlet flowrates faster than 5 L/h. Mixing performance is even better when these devices have a separate mixing chamber upstream of a narrower outlet pipe, whose diameter is half that of the chamber. Computational Fluid Dynamics shows the internal flow depends on the geometry of the mixer and has a direct influence on its efficiency. When no separate chamber is present, the flow has a double spiral structure. The two inlet fluids roll up together as they pass through the mixer, which slows down the mixing process. For precipitation reactions, we recommend the use of a mixer with conical narrowing which has a high mixing efficiency and a lower risk to be obstructed by the particle agglomeration. (C) 2015 Elsevier B.V. All rights reserved.