An experimental setup has been developed and applied for the combined determination of the electrokinetic potential and the surface conductivity of flat surfaces. The key feature of the new device (designated as microslit electrokinetic setup) is the variability of the distance between two parallel flat sample surfaces (10 mm x 20 mm) forming a slit channel. The setup allows us to decrease this distance down to about 1 µm keeping the surfaces parallel. In consequence, streaming potential measurements can be performed at a given solid/liquid interface both at conditions where surface conductivity is negligible and at conditions where surface conductivity significantly contributes to the total channel conductivity. The zeta potential is calculated at different channel geometries based on streaming potential and channel conductivity data and, alternatively, based on streaming current measurements and the dimensions of the cross section of the slit channel. The results obtained were found to agree well if correct conductivity values for the calculation of the zeta potential based on the streaming potential data are used. The surface conductivity is determined from the extrapolation of the channel conductance values gained at a number of sufficiently small distances between the parallel sample surfaces to the distance zero. An additional feature of the developed microslit electrokinetic setup is the assessability of the hydrodynamic thickness of adsorbed layers of macromolecules or particles at the investigated flat surface. In a series of measurements a plasma-deposited fluoropolymer (PDFP) layer on top of a glass carrier and an adsorption layer of the blood protein fibrinogen on top of the PDFP layer were characterized by zeta potential and surface conductivity measurements in different aqueous electrolyte solutions (KCl, KOH, HCl). For the PDFP/solution interfaces zeta potential up to -100 mV were obtained in solutions of neutral pH exclusively due to preferential ion adsorption. After adsorption of fibrinogen the zeta potential is considerably reduced. For the PDFP/solution interfaces surface conductivities were determined in the range of (1-2) x 10(-9) S. The contribution of the diffuse layer to the surface conductivity has been calculated from the zeta potential according to the approach of Bikerman (Kolloid Z. 72, 100 (1935)) and compared with the experimentally determined surface conductivity. Based on this comparison ions in hydrodynamically immobile interfacial layers were concluded to contribute considerably to the surface conductivity in all investigated cases. This so-called additional surface conductivity is attributed to the accumulation of hydroxide and hydronium ions in the Stern layer. Both the high specific mobility of these ions (as compared to the potassium and the chloride ions) and the conductivity of the charge determining species may contribute to the experimental observations. After adsorption of fibrinogen onto the PDFP surface the additional surface conductivity is increased by about an order of magnitude. The latter fact is assumed to be caused by the presence of mobile ions in the interfacial volume of the adsorbed protein layer. In addition to the electrochemical characterization of the adsorbed protein layer its hydrodynamic thickness has been determined by means of liquid flow measurements with the microslit electrokinetic setup. The obtained value of 48 +/- 5 nm correlates well with the protein dimensions given in the literature and is in the order of magnitude of the optical layer extension determined by ellipsometry. Copyright 1998 Academic Press.