A number of factors influence the efficiency and the economics of the separation of dispersed liquid drops in an immiscible liquid–liquid mixture. One important factor that controls the performance of a filter medium in the separation is the fiber size. Electrospun polymer nanofibers have diameters in the nanometer range and are arbitrarily long. The experimental results in this work show that adding nanofibers to conventional micron-sized fibrous filter media improves the separation efficiency of the filter media but also increases the pressure drop. An optimum in the performance occurs (significant increase in efficiency with minimal increase in pressure drop) with the addition of about 1.6% by mass of 250 nm diameter nylon 6 nanofibers to 5 micron diameter glass fibers in the filters. © 2005 American Institute of Chemical Engineers AIChE J, 2005