The purpose of this study was to find the primary factor determining the mixing of the counter anions of 1-alkyl-3-methylimidazolium-based ionic liquid (IL, C n mimX) at the air/water interface. First, the properties of the adsorbed films of the C 1 0 mimBr-C 1 0 mimBF 4 mixtures were estimated by using the thermodynamic analysis of their surface tension data. The mole fraction of BF 4 - was lower in the adsorbed films than in the ideal state and the excess Gibbs energy of adsorption was negative, which was in marked contrast to those of the C 6 mimBr-C 6 mimBF 4 mixtures. Meanwhile, the possibility of forming side-by-side ion pair of BF 4 - and imidazolium ring at the adsorbed films, as we found in the C 6 mimBr-C 6 mimBF 4 system, was greatly reduced in the present case. These changes were mainly attributed to the reduction in the hydrogen-bonding interaction between BF 4 - and C 1 0 mim + , as the carbon number in the hydrophobic part of C n mim + was increased from 6 to 10. In addition, it was found that the values of excess Gibbs energy of adsorption were less negative in the C 1 0 mimBr-C 1 0 mimBF 4 system than in the dodecyltrimethylammonium (C 1 2 TA)Br-C 1 2 TABF 4 system, where the long-range Columbic interaction solely exists between the counter anions and surfactant cations. Thus, we concluded that the short-range hydrogen-bonding interaction played an essential role in the interfacial activity of ILs.