Abstract: The interaction between 1-butyl-3-methylimidazolium hydrosulphates ionic liquid (BmimHSO₄) and nitrogenous compounds (quinoline and indole) were investigated by density functional theory, and analyzed by NBO and AIM methods. The most stable structure of BmimHSO₄ ionic pair displays a strong hydrogen bond interaction between O atom in HSO₄^- anion and C14-H20 in imidazolium ring. NBO and AIM analysis at the molecular level proves that there is a strong interaction between quinoline or indole and the anion of BmimHSO₄. The strong interaction between the active hydrogen of HSO₄^- and the N atom of quinoline molecule as well as the hydrogen bond formed between the H atom of N-H in indole molecule and the O atom of HSO₄^- are the main driving force for denitrogenation performance of BmimHSO₄.