The critical micelle concentration (cmc) of sodium dioctylsulfosuccinate (ACT) was determined at 25 C from surface tension and fluorescence methods in aqueous NH4Cl solution for assessing the influence of mixed counterions on the special counterion binding behavior (SCB) of ACT. The SCB of AOT refers to a sudden twofold increase in the value of the counterion binding constant (beta) in aqueous medium when the concentration (C) of the added 1:1 sodium salt is about 0.015 mol kg(-1), and it has been tested so far for sodium ion only. In the presence of sodium and ammonium mixed counterions also the SCB of ACT exist, but with lower c(+) (0.009 mol kg(-1) NH4Cl). Synergism in the cmc occurs due to mixed counteHons. In the case of inorganic counterions, unlike the case with organic counterions, the cmc is dependent on the total counterion concentration in solution and negligibly on the specific type of counterion. Na+ and NH4+ bind almost equally to the micelle in the region of low If (below c'), but in the region of high beta (above c(+)) NH4+ binds predominantly. It has been shown that the theoretical expression for the surface excess of ionic surfactant + electrolyte system containing a single counterion can also be used to evaluate the surface excess in the presence of mixed counterions if the two counterions are considered to undergo Henry-type adsorption at the air-solution interface. (C) 2011 Elsevier Inc. All rights reserved.