Oil-in-water model emulsions, encompassing four mineral oils of different properties, were separated using polyurethane and polyethylene terephthalate fibers. Experiments were carried out over a wide range of oil properties, bed permeability, and fluid velocities. The objective of the work was to introduce a novel approach for the estimation of the efficiency of fiber bed coalescence using contour diagrams and a new variable, namely iso-critical velocity. The simultaneous effect of a wide range of dispersed oil properties, fiber bed permeability, and nature of polymeric fibers on the iso-critical velocity was investigated. The novel approach gives the possibility to detect regions of optimal bed permeability, dispersed oil properties and critical velocity. The results indicate higher iso-critical velocity for the polyurethane fiber bed coalescer over a broad range of working conditions. The presented contour diagrams may also serve to determine stable working parameters, ensuring a desired separation efficiency and effluent oil concentration. (C) 2012 Elsevier B.V. All rights reserved.