Fiber-bed coalescers can be used to separate liquid-liquid dispersions up to the solubility limit. Until now, their design is still based on empirical data and expertise. To facilitate their design, the development of a physically founded model describing the processes in a fiber-bed coalescer is currently in progress. The most important design criteria are pressure drop and separation efficiency of the fiber bed. A successful new attempt to describe the two-phase pressure drop has been made. For the first time, the structure of the two-phase flow inside a fiber bed could be observed directly be means of a technical endoscope. (C) 2002 Published by Elsevier Science B.V.