The zeta-potential of a solid-liquid interface is an important surface characterization quantity for applications ranging from the development of biomedical polymers to the design of microfluidic devices. This study presents a novel experimental technique to measure the zeta-potentials of flat surfaces. This method combines the Smoluchowski equation with the measured slope of current-time relationship in electroosmotic flow, This method is simple and accurate in comparison with the traditional streaming potential and electrophoresis techniques. Using this method the zeta-potentials of glass and poly(dimethylsiloxane) (PDMS) coated surfaces in KCl and LaCl3 aqueous solutions were measured using several flow channels ranging from 200 to 300 mum in height. The zeta-potential was found to vary from -88 to -66 mV for glass surface and -110 to -68 mV for PDMS surfaces depending on the electrolyte and the ionic concentration. The measured values of the zeta-potential are found to be independent of the channel size and the applied driving voltage and generally are repeatable within +/-6%. (C) 2003 Elsevier Science (USA). All rights reserved.