In a 4-year laboratory bench study of coalescence of free water in JP-5 and other fuels with added corrosion inhibitors and strong surfactants, measurements were made of contact angles on surfaces submerged in fuel, water-fuel interfacial tensions, capillarity and porosity of coalescer media, and of the attachment, growth, and detachment of water on single fibers in a flowing suspension. Many coalescer media were tested in small cells, both under water separometer operating conditions and in larger cells at realistic mat depths and flow velocity. Tests of individual media are meaningless, for effective performance is obtained only in composite mats with layers of varying structure. Certain experimental mats were far superior to plugs cut from commercial filtercoalescer elements. Antiicer greatly reduced the deleterious effect of surfactants. Water removal from diesel fuel is difficult, but may be done by using two coalescers in tandem.