The　vapor　pressures　of　the　binary　systems　(water　+　[OMIM][BF4],　water　+　[OMIM][Tf2N],　and　water　+　[OMIM][PF6])　were　measured　by　a　modified　equilibrium　still　at　temperatures　ranging　from　323.15　to　358.15　K　and　water　mole　fraction　ranging　from　0.1　to　0.9　for　water　+　[OMIM][BF4],　from　0.1　to　0.4　for　water　and　[OMIM][Tf2N]　and　from　0.05　to　0.45　for　water　+　[OMIM][PF6]　binary　systems,　respectively.　The　experimental　results　indicate　that　with　the　addition　of　ionic　liquids　(ILs)　the　vapor　pressure　of　binary　systems　decreases.　In　addition,　the　popular　and　widely　used　UNIFAC-Lei　model　was　extended　to　the　corresponding　systems　to　predict　the　vapor　pressures　with　the　average　relative　deviations　(ARDs)　of　4.99%,　4.71%　and　5.15%　for　water　+　[OMIM][BF4]/[OMIM][Tf2N]/[OMIM][PF6]　systems,　respectively.　Moreover,　the　binding　energy,　excess　enthalpies　and　s-profiles　were　investigated　to　analyze　the　thermodynamic　behavior　at　the　molecular　level.　It　was　found　that　the　interaction　between　[OMIM][BF4]　and　water　is　significantly　higher　than　those　between　[OMIM][Tf2N]/[OMIM][PF6]　and　water　as　shown　by　the　sigma-profiles　and　the　binding　energy,　and　the　hydrogen　bonding　interaction　is　the　dominant　driving　force　contributing　to　the　overall　excess　enthalpy.　(C)　2020　Elsevier　B.V.　All　rights　reserved.