The effect of SDS on the electrokinetic behavior of TiO2 and Al2O3 was studied by electrophoresis at various solid-to-liquid ratios. Additionally, the effect of CTAB on electrokinetic curves of Al2O3 single crystal and of Al2O3 particles was studied by streaming potential. At a sufficiently low solid-to-liquid ratio, the electrokinetic potential was negative and almost pH-independent in the presence of SDS and positive and pH-independent in the presence of CTAB. Further decrease in the solid-to-liquid ratio had a limited effect on the course of the electrokinetic curves. At a sufficiently high solid-to-liquid ratio, the electrokinetic potential was not affected by the presence of the surfactant. At moderate solid-to-liquid ratios, the electrokinetic potential in the presence of SOS was negative and almost pH-independent at very high and at very low pH, and less negative or even positive electrokinetic potential (more positive at higher solid-to-liquid ratios) was observed at moderate pH with a peak 1 to 2 pH units below the pristine IEP. The inspection of the results (obtained at single solid-to-liquid ratio) from the literature confirmed the above trends, also for oxides other than TiO2 or Al2O3. The range of solid-to-liquid ratios, which can be covered by electrophoresis is limited by insufficient signal, and by insufficient transparency at low and at high solid-to-liquid ratios, respectively. The available range of solid-to-liquid ratios can be extended by using the electroacoustic method. Apparently, the significance of the solid-to-liquid ratio in the electrokinetic studies of oxide-ionic surfactant systems is underrated. To our best knowledge, this is the first systematic study of such an effect ever published, and in many publications, the solid-to-liquid ratio was not reported and probably not even controlled. (C) 2012 Elsevier Inc. All rights reserved.