Separation efficiency reaches a maximum dependent upon the level of flocculant solution-substrate mixing for a fixed flocculant dose. At low levels of mixing poor separation is due to poor distribution of the flocculant molecules. At high levels of mixing floc attrition occurs which reduces the separation efficiency. On either side of this maximum conditions leading to similar sedimentation rates can be identified. At these similar sedimentation rates lower sediment volume and supernatant turbidity are obtained with flocs which have undergone a degree of breakage. The unique molecular architecture (UMA(R)) type of flocculant was found to be superior to a standard flocculant at several levels of mixing. This superior performance was explained by relative ease of distribution of flocculant molecules in the suspension and by greater floc strength. However, differences in floc density did not explain performance differences. Evidence for the concept of "undermixing" is presented, which can explain poor solid-liquid separation performance which otherwise could appear to be explained by flocculant overdosing. (C) 2002 Elsevier Science Ltd. All rights reserved.