Ionic liquids (ILs) have been suggested as useful extractants of aromatic nitrogen-containing compounds (N compounds) from fuel oils. In this systematic study, ILs based on common cations and anions are employed as extractants of the archetypical N compounds pyridine and indole from a model oil consisting of decane and toluene. The performance of these ILs as extractants of N compounds is compared and rationalized. It is demonstrated that the cation and anion sizes (offering more surface area for extractants to interact) are the major factors determining the effectiveness of N compound extraction, although hydrogen bond donor/acceptor abilities of ILs can also play a role in the removal of these N compounds. In this study, some ILs are found to dissolve a considerable amount of oil contents. This undesired property can be controlled by the size of IL ions.