At 298 K, the surface tension of ionic liquids (ILs) of the 1-alkyl-3-methylimidazolium bis- (trifluoromethylsulfonyl)imide series, [CnC1,Im][NTf2 ], ranges 36 from around 35 mN center dot m(-1) for [C2C1 Im][NTf2] to just below 30 mN center dot m(-1)for [C(12)C(1)Im][NTf2]. However, the decrease rate along the series is not constant: a large decrease from [C(2)C(1)Im][NTf2] to [C(8)C(1)Im][NTf2] is followed by almost constant values from [C(8)C(1)Im][NTf2] to [C(12)C(1)Im][NTf2]. Such behavior is hard to interpret from a molecular point of view without suitable information about the free-surface structure of the different ILs. In this work, we have successfully used the Langmuir principle in combination with structural data obtained from angle-resolved Xray photoelectron spectroscopy experiments and molecular dynamics simulations, to predict the correct surface tension trend along the IL series. The concepts unveiled for this particular homologous IL family can be easily extended to other systems.