The aggregation of two imidazolium-based ionic liquids, 1-ethly-3-methylimidazolium chloride (EMI(+)Cl(-)) and EMI(+) bis-(trifluoromethanesulfonyl)amide (EMI(+)TFSA(-)), in molecular liquids, water, methanol, acetonitrile, and benzene, has been studied by using the small-angle neutron scattering (SANS) technique. The SANS results have shown that the heterogeneity of EMI(+)Cl(-)-acetonitrile mixtures is significant at high acetonitrile contents, thus, EMI(+)Cl(-) forms clusters in acetonitrile solutions. On the other hand, it has been revealed that EMI(+)Cl(-) is homogeneously dissolved in water and methanol. EMI(+)TFSA(-) remarkably aggregates in methanol solutions, while the mixtures of EMI(+)TFSA(-) with acetonitrile and benzene are homogeneous. Furthermore, aggregation of EMI(+)Cl(-) and EMI(+)TFSA(-) in acetonitrile and methanol, respectively, has been examined by using (1)H NMR spectroscopy. The mechanism of aggregation of the ionic liquids in the molecular liquids has been discussed on the basis of the properties of cations, anions, and molecular liquids.