A new membrane technique was successfully used for separating the two phases of an industrial emulsion from the aluminum industry. Unlike other membrane processes, the membrane acts here as a coalescer. Demulsification occurs when the emulsion is permeated through some microfiltration membranes. After the permeate has been allowed to settle for 2 h, the oil concentration in the lower layer can be as low as 30 ppm compared to 30,000 ppm in the original emulsion. The effects of cross-flow velocity, pressure and temperature were investigated on a pilot plant fitted with a 0.2 mu m microfiltration module of 1 m(2) filtration area. Operating at a temperature of 40 degrees C and using a low transmembrane pressure of 30 kPa gave a flux of 80 I/h m(2), irrespective of the cross-flow velocity. The process proved reliable during 4-week operation.