The relationship between the measured electrostatic surface potential of two mixed surfactant systems, n-dodecyl octaoxyethylene glycol monoether (C12E8)/Sodium dodecyl sulfate (SDS) and C12E8/n-dodecyltrimethylammonium chloride (DTAC), and the free ionic surfactant concentration in these mixtures has been examined. The surface potential measurements were based on the well established technique of using micelle-solubilized acid-base indicators. Free monomer concentrations were determined from results obtained using the mixed micelle critical micelle concentrations and then processing this information using a nonideal mixed micelle theory. For the systems studied it has been found that a change in the modulus of the surface potential per unit change in log(ionic surfactant concentration) is approximately 59 mV. This is in accordance with the Nernstian relationship put forward by Healy et al.1 linking the electrostatic surface potential of a charged micelle to the equilibrium monomer concentration.