We present measurements of the variation of mass suffered by eight different ionic liquids (ILs) according to the atmosphere humidity grade. The ILs studied belong to two different families, thus four of them belong to the 1-alkyl-3-methyl imidazolium tetrafluoroborate (CnMIM-BF4) family, and the other four to the 1 -ethyl-3-methyl imidazolium alkyl sulfate (EMIM-CnS) one. This allows us to study the influence of the alkyl chain length on the water absorption from atmosphere. The results indicate that the water is physio-adsorbed in the free surface of the sample when the humidity grade increases, and later this is desorbed again when the atmosphere is dry. In the CnMIM-BF4 family desorption is complete, meanwhile in I he EMIM-CnS one some residual adsorbed water remains. In addition, we have studied the influence of the free IL surface size in the adsorption process. Finally, we left two ILs (one of each family) and an equimolar aqueous solution of one of them during more than 70 h to observe adsorbed water saturation (C) 2009 Elsevier B.V. All rights reserved.