Fourier-transform infrared and Raman spectroscopies have been used to investigate the role played by water on the structural organization of 1-butyl-3-methyl-imidazolium tetrafluoroborate and H2O mixtures, over a wide composition range at room temperature. Our measurements provide clear experimental evidence of the prompt association between ‘free’ water molecules and anions at very small water contents. Moreover, in the case of higher water contents, we obtain indications of the local aggregation of water molecules in the network. Such an aggregation is found to occur even before the saturation of all the anions that are available for H-bonding. We propose that the water clusters favour the organization of ionic liquid in the polar network, where they are embedded, and the aggregation of hydrophobic alkyl tails in a micelle-like structure. When mixtures with water molar fraction exceeding 0.7 are considered, this local organization starts to weaken owing to the gradual break up of the ion-pair interactions. Copyright © 2007 John Wiley & Sons, Ltd.