A practical study is presented of the effect of height/diameter (H/D) ratio of settlers on the kinetics of phase separation in aqueous two-phase systems (ATPSs). Poly(ethylene glycol) (PEG) 1000-phosphate systems with the presence of undiluted and diluted whey and disrupted yeast were used in batch studies. The influence of the biological suspensions on the rate of phase separation was investigated. It was observed that, phase separation time is much faster when disrupted suspension was used. The addition of undiluted suspensions to ATPSs slowed the process of phase separation. When the batch settler with a large cross section area (H/D ratio less than one) was used, the phases separated much faster than in a settler with a H/D ratio greater than one. Conclusions are drawn concerning the characterisation of the process and design parameters involved in the phase separation for the design of appropriate equipment. (C) 2000 Elsevier Science B.V.