Sepiolite is a hydrated magnesium silicate with a fibrous structure. It undergoes acid-base interactions around the natural pH of 8.5. The zeta potential (zp) of sepiolite depends on the type, the chemical, and the mineralogical composition of sepiolite and more importantly the ionic environment of the suspensions. In this study, electrokinetic properties of sepiolite have been determined. A series of systematic zeta potential measurements have been performed to determine the isoelectric point (iep), the potential determining ions (pdi), and the effect of monovalent ions on the zeta potential of sepiolite. The electrokinetic behavior of sepiolite was examined in the presence of monovalent ions to identify if they adsorb through ion exchange or get incorporated in the diffused layer. A schematic ion adsorption/ion exchange model which illustrates the arrangement of monovalent ions in the electrical double layer of sepiolite is proposed.