The effect of pH and crossflow velocity on the ultrafiltration of a model metalworking o/w emulsion was studied. The emulsion was prepared using a vegetable oil as the base oil and a mixture of anionic and non-ionic surfactants as emulsifiers. Experiments were carried out using Carbosep((R)) tubular ceramic membranes with two different cut offs (50 and 300 kDa) in a transmembrane pressure range of 0.05-0.4 MPa. Concentration polarization was observed at low crossflow velocities. Zeta potential and droplet size distribution measurements showed that properties of the o/w emulsion are not affected by pH. However, both permeate flux and chemical oxygen demand (COD) retention decreased drastically at low pH values, since membranes become positively charged and adsorption of anionic surfactants onto the membrane surface takes place, making it more hydrophobic and causing flux decline and surfactant monomers permeation through the membrane. The resistance-in-series (RIS) model was applied in order to ascertain the main resistances to the permeate flux. COD retentions higher than 92% were achieved for all the operating conditions with both membranes. (c) 2005 Elsevier B.V. All rights reserved.