The effect of an external electric field on the flux in crossflow membrane filtration of a model oily waste water was studied using a carbon fibre - carbon composite membrane as a cathode. Limiting fluxes for low flow rate increased significantly under the conditions studied, from 75 l/m(2) h without an electric field to more than 350 l/m(2) h using an electric field. The experimentally determined increase in the limiting flux showed good agreement with the theoretical value of 430 l/m(2) h calculated using a simple model. The limiting flux increase was affected by the electrophoretic mobility of the oil droplets and the applied electric field strength. When there were no cakes without an electric field due to the high flow rate, the flux increase when using an electric field under at the same conditions was minor. The critical electric field strength was determined, and experimentally obtained values were corresponded with calculated, values. Decreasing the crossflow velocity above the critical electric field strength increased the flux, or had no effect, depending on the size of the particles. Permeate quality was also improved to some extent when using the electric field, and a membrane with a large pore size could be used when an electric field was applied. The main disadvantage in using the membrane as a cathode was foaming at the membrane surface causing decrease in the flux enhancement as the conductivity of the feed increased. It was not possible to restore the flux to the original value by applying an electric field after filtration of the oil emulsion without an electric field. An intermittent electric field was thus not efficient enough for keeping the flux at high level.