High-turbidity waters, containing up to a few percent of suspended solids, occur quite widely and present considerable treatment problems, especially because of low sedimentation rates. Such waters are normally treated by sedimentation and the rate can be greatly increased by the use of polymeric flocculants. The degree of flocculation achieved and the improvement in settling rate can be markedly affected by the dosing and mixing conditions. These effects have been studied with stirred clay suspensions, using a simple, flow-through monitor which gives a rapid indication of the degree of flocculation. The same monitor has been used to derive sedimentation rates, which show good correlation with the flocculation response. For high solids concentrations and relatively low polymer doses, flocculation occurs rapidly, but the flocs are not stable and can be broken at moderate stirring rates. This leads to a maximum floc size and a corresponding maximum settling rate soon after dosing the suspension with polymer. This peak is most pronounced with the most concentrated suspensions. By reducing the stirring rate after a short period, the flocs can be maintained at their peak size, without break-up. In this way, appreciable flocculation can be achieved with much lower polymer dosages (about 100 μg/g, based on the weight of solids) than are usually reported.