In this paper, a mechanical coalescer is tested to coalesce the oil droplets in the oil-in-water (O/W) emulsions. The coalescence of the O/W emulsions through fibrous and granular beds is studied. More specifically, the pressure drop across the coalescing beds is successfully predicted by using the Carman-Kozeny equation for the single-phase flow. A geometric method for the fibrous beds and an experimental method for the granular beds are used to determine the Kozeny constant, respectively. For the two-phase flow, the pressure drop is slightly increased as compared with that for the single-phase flow. In addition, the effects of flow rate, inlet oil concentration, length of the coalescing bed and fiber size on the coalescence performance are examined. In the parametric studies, first, it has been found that the overall coalescence efficiency is not a monotonous function of flow rate in the granular beds. The coalescence efficiency is higher at some intermediate superficial velocities. Secondly, the higher the inlet oil concentration is, the lower the overall coalescence efficiency is. Thirdly, the coalescence performance is insensitive to the length of the granular bed tested in this study. It has also been found that the coalescence performance is better for a fibrous bed packed with smaller fibers. (C) 2004 Elsevier B.V. All rights reserved.