The properties of mixed surfactants of 1-tetradecyl-3-methylimidazolium bromide (C(14)mimBr) and the gemini surfactant N,N'-bis(dimethyldodecyl)-1,2-ethanediammonium dibromide (12-2-12) in water and in ethylammonium nitrate (EAN) were studied by surface tensiometry. The critical micelle concentration (cmc) values of the mixed surfactants were determined and found to be much larger in EAN than in water. The nonideal mixing behaviors were observed at the air/solvent interface and in the mixed micelle both in water and in EAN. The surface pressure at the cmc (pi(cmc)), the maximum surface excess (G(max)), the minimum surface area per molecule (A(min)), the standard Gibbs free energies of micellization (Delta G(m)(0)) and adsorption (Delta G(ads)(0)), and the excess Gibbs free energy of mixed micelles (Delta G(E)) were determined. It was found that physicochemical properties of C(14)mimBr/12-2-12 mixed systems in water and in EAN were significantly different, which was discussed in terms of different properties of the solvents.