An ion-exchange process has been used to prepare HOT from NaOT (sodium bis(2-ethylhexyl)sulfosuccinate), where the Na+ counterion has been replaced by H+. The acidity function, Ho, of the aqueous core of HOT-based microemulsions decreases with W from H-0 approximate to 0.6 at W > 20 to H-0 = -1.4 at W = 2. On the basis of the Ho acidity function of the aqueous core and the dependence of Ho on acid concentration, we have been able to determine the degree of counterion binding (beta) in microemulsions with a value of beta approximate to 0.92 which is practically independent of the water content of the system. (C) 2007 Elsevier Inc. All rights reserved.