Coalescence filtration is very effective for the separation of secondary emulsions that contain water droplets with diameters less than 50 mum. The factors that control the performance of coalescer filter media are fiber size and wettability. High wettability materials for water-in-oil dispersion promote coalescence. New experimental results investigating the performance of nonwoven filter media glass fiber augmented with polymer nanofiber are presented in relation to the relevant parameters (wettability, filter depth, flow velocity, and filter materials). A decrease in fiber size improves the overall separation efficiency of the process. However, the nanofibers cause a significant increase in the pressure drop through the filter. (C) 2004 American Institute of Chemical Engineers.