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• The structural effects on VLE of H 2 O-IL systems was experimentally investigated. • The UNIFAC-Lei model was well extended for predicting VLE of H 2 O-IL systems. • HB interaction mechanisms in H 2 O-IL systems at the molecular level was identified. • The relationship between the VLE behaviors and the HB interactions was explored. The relationships between the structure and the thermodynamic behavior and the hydrogen bond (HB) interactions in H 2 O + 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 ([BF 4 ]-), and bis (trifluoromethylsulfonyl)imide ([Tf 2 N]-) were investigated. The vapor–liquid equilibrium (VLE) of H 2 O + 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. [ABSTRACT FROM AUTHOR]