To investigate the counterion effects, we employed dodecyltrimethylammonium bromide (DTABr)-dodecyltrimethylammonium tetrafluoroborate (DTABF(4)) mixed aqueous solutions and measured their surface tensions, then analyzed these data in a thermodynamic treatment. The tensiometry showed that DTABF(4) was more effective in lowering the surface tension of water. The phase diagram of adsorption demonstrated that the surface was enriched with BF(4)(-) ions, but the composition of Br(-) ions in the adsorbed film was slightly enhanced compared to the ideal mixing criteria. These were explained in terms of the size and polarizability of counterions. Moreover, the distribution of counterions of the DTABr-DTABF(4) mixtures in the adsorbed film is greatly different from that of the 1-hexyl-3-methylimidazolium bromide (HMIMBr)-1-hexyl-3-methylimidazolium tetrafluoroborate (HMIMBF(4)) mixtures, where a stronger hydrogen-bonding exists between BF(4)(-) and HMIM(+) ions. These findings suggest that the adsorption of counterions in electric double layers is likely subject to two factors: the nature of counterion and their interactions with surfactant ions. (C) 2011 Elsevier Inc. All rights reserved.