The low throughput of the disperse phase is one of the issues in cross-flow membrane emulsification. This is apparent in the low percentage of pores at which droplets are formed (few active pores). To determine the effect of membrane morphology on pore activation, we developed and experimentally validated a model that describes the flow phenomena in and under a membrane with uniform pores (microsieve). In this model the membrane is divided into two parts: the toplayer and the membrane substructure. The model was validated with a larger-scale physical analogon. It predicts a linear increase of the number of active pores with increasing transmembrane pressure, while the pressure difference over the active pores is independent of the transmembrane pressure as long as not all pores are active. Although the resistance of the microsieve substructure was found to be four times lower than the resistance of a single pore, the resistance of the membrane substructure had a large effect on the activation of pores. Hence, the number of active pores can be increased by increasing the ratio of flow resistance in the pores and the flow resistance in the membrane substructure. Preliminary experiments show that the gradual increase in active pores at a ceramic membrane surface can be explained in the same way. (C) 2003 Elsevier Science B.V. All rights reserved.