In order to optimise the membrane emulsification process, there is a need for a model that can be used to keep the production time of en emulsion to a minimum, without impairing its quality. This was the aim of our work. For that, we have studied the influence of transmembrane pressure on the fraction of active pores and on the drop diameter and the drop diameter distribution. We have found that an increase in transmembrane pressure makes it possible to increase the dispersed phase flux by a rise in the number of active pores (from 1 to 13% for the 0.2 mu m membrane and from 2 to 45 and 49% for the 0.5 and 0.8 mu m membranes, respectively). Furthermore, the increase in transmembrane pressure leads to an increase in the diameter of the droplets produced. For the 0.5 and 0.8 mu m pore diameter, we have even found an increase in the polydispersity of the emulsion produced. A thorough study allowed us to locate the origin of this polydispersity: we ruled out post-production coalescence both in the bulk and in the laminar sub-layer. We showed the possibility of the expansion of the three-phase contact line over more than one pore, phenomenon we referred to as membrane coalescence. Based on these results, we built a model which allows to evaluate the risk of membrane coalescence under experimental conditions of dispersed phase flux and transmembrane pressure: the number of active pores at a distance d(d)/2 from the centre of a given active pore must be <2. For the three pore diameter membranes studied, our model matched the experimental results and could explain the polydispersity observed. (C) 2010 Elsevier B.V. All rights reserved.