For small drops dispersed in another liquid with a small density difference, gravitational separation is not effective and other mechanical methods such as centrifugation are expensive. An attractive way to separate the dispersed phase is by the use of electrical phenomena, whereby the differences in conductivity and dielectric properties between the two phases can be exploited to generate significantly high surface forces, where under favourable conditions they can enhance electrocoalescence. In this paper, the design and operation performance of two novel compact electrocoalescer-separators are described. The application of an external electric field has been shown to significantly enhance the separation of aqueous drops in a flowing viscous oil, with a low concentration of the dispersed phase. The separation efficiency increases with the electric field strength until a limit, above which drop deformation and break-up occur. Using pulsed direct current (dc) electric fields, an optimum electric field and frequency is found for the enhancement of drop-drop and drop-interface coalescence, thus producing a maximum separation efficiency for each of the two separators. The separation efficiency increases with inlet drop size up to a certain diameter. Above this diameter, the drops deform and short-circuit the electrodes easily. Short-circuiting can be minimised by optimising the electric field. Larger throughputs of the aqueous and oil phases are obtained with the electrocoalescer-separators that uses combined electrical and centrifugal effects, compared with the electrocoalescer-separator that is based on electrical and gravitational effects. The novel compact electrocoalescer-separators can be easily installed on-line to pipelines for treating water-in-oil dispersions occurring in the etude oil and petroleum industries. (C) 2002 Elsevier Science B.V. All rights reserved.