The　relationships　between　the　structure　and　the　thermodynamic　behavior　and　the　hydrogen　bond　(HB)　interactions　in　H2O　+　ionic　liquid　(IL)　systems　were　systematically　investigated　for　the　first　time.　Imidazolium-based　ILs　comprising　1-alkyl-3-methylimidazolium　cations　([RMIM]+)　and　various　anions　such　as　acetate　([AC]-),　thiocyanate　([SCN]-),　tetrafluoroborate　([BF4]-),　and　bis(trifluoromethylsulfonyl)imide　([Tf2N]-)　were　investigated.　The　vaporliquid　equilibrium　(VLE)　of　H2O　+　IL　systems　was　experimentally　determined　and　predicted　by　the　UNIFAC-Lei　model.　The　results　demonstrated　that　the　vapor　pressure　is　mainly　dependent　on　the　type　of　anion　and　increases　slightly　with　an　increase　in　the　cation　alkyl　chain　length.　Quantum　chemistry　calculation　and　wavefunction　analysis　clearly　revealed　the　relationship　between　the　VLE　and　the　HB　interactions.　This　study　provides　theoretical　guidance　for　designing　and　screening　task-specific　ILs　for　chemical　processes　such　as　gas　dehydration　and　dehumidification.　©　2020　Elsevier　Ltd