Quantum chemical calculations have been used to investigate the interaction between water molecules and ionic liquids based on the imidazolium cation with the anions [Cl-], [Br-], [BF4-], and [PF6-]. The predicted geometries and interaction energies implied that the water molecules interact with the Cl-, Br-, and BF4- anions to form X-center dot center dot center dot W (X = Cl or Br, W = H2O), 2X(-)center dot center dot center dot 2W, BF4-center dot center dot center dot W, and W center dot center dot center dot BF4-center dot center dot center dot W complexes. The hydrophobic PF6- anion could not form a stable complex with the water molecules at the density functional theory (DFT) level. Further studies indicate that the cation could also form a strong interaction with the water molecules. The 1-ethyl-3-methylimidazolium cation (Emim(+)) has been used as a model cation to investigate the interaction between a water molecule and a cation. In addition, the interaction between the ion pairs and the water was studied by using 1-ethyl-3-methylimidazolium chloride (Emim center dot Cl) as a model ionic liquid. The strengths of the interactions in these categories follow the trend anion-W > cation-W > ion pair-W.