In this article we assume that each charged particle in a solvent is characterized by two radii: a geometric and an electrokinetic radius, which correspond to two electric potentials: geometric and electrokinetic potential, respectively. We derived equations by means of mathematical analysis that represent the electrokinetic radius as a function of the ionic strength of the solution where the charged particle is located. The displayed equations can be used to calculate the zeta-potential of ions and polyions at different ionic strengths. (C) 2013 Elsevier Ltd. All rights reserved.