The electrophoretic mobility of five different-sized spherical colloidal gold particles has been measured in aqueous potassium chloride (KCl) and sodium phosphate electrolyte solutions over the concentration range 0.005-0.154 M solutions. The measured mobilities are independent of electrolyte type and dependent on electrolyte concentration. Interpretation of the mobilities using the simplified electrokinetic formula of Ohshima (J Colloid Interface Sci. 2001, 239, 587-590) indicates that the magnitude of the effective electrokinetic charge density and total charge of particle remains apparently constant with electrolyte concentration. The effective electrokinetic surface charge is negative in KCl and sodium phosphate solutions. The zeta-potential shows a maximum absolute value in both electrolyte solutions (negative).