This paper presents results on membrane coalescence of stable oil-in-water emulsions. The method involves a two-stage process in which the membranes are used to enlarge oil droplets such that a traditional gravity settler can be used for oil phase separation. Several factors that affect the performance of the membrane coalescer have been studied. In particular, the effect of membrane pore size and the imposed in-pore shear rates on coalescence of stable micron and submicron oil droplets present in oil-in-water emulsions were investigated. Hydrophobic membranes made of Teflon (PTFE) with nominal average pore sizes of 0.22, 0.45, 1.2 and 5.0 mum were tested to assess their ability to coalesce oil emulsions with an average d(50) 1.5 mum droplet diameter (volume based). Experiments were carried out in a batch dead-end filtration cell and the influence of operating conditions such as transmembrane pressure, membrane orientation, and emulsion concentration were investigated. The coalesced filtrate was allowed to settle for I h, and the percentage oil removal achieved for 0.22, 0.45, 1.2 and 5.0 mum was up to 86, 81, 66 and 45%, respectively. The results showed that membrane pore size had a large influence on the coalescence of oil droplets and that membrane orientation is important. Also, imposed shear rates inside the membrane pores play a key role during membrane coalescence, where sufficient in-pore shear rates are required to coalesce oil droplets, although higher shear rates may reverse the process. (C) 2003 Elsevier B.V. All rights reserved.