The formation of drops is a topic of great interest in a wide variety of engineering applications, such as membrane emulsification. in order to develop an improved force balance model that is capable of predicting the final size of the detached drop, the formation of drops into a cross-flowing continuous phase has been studied with computational fluid dynamics (CFD). The force balance model developed takes into account the drop deformation that occurs as the drop approaches detachment. The results given by the model have been compared with CFD simulations, and the drop diameters agree within 10%, except at low wall shear stresses. The model has also been compared with experimental results on drop formation using various membranes, cross-flow velocities and surfactants. The difference between the model and experimental results is mainly due to the adsorption of surfactants onto the drop interface and the shape of the membrane pores. (C) 2009 The Institution of Chemical Engineers. Published by Elsevier B.V All rights reserved.