Imidazolium ionic liquids (ILs) can be functionalized by introducing -NH2, which were found to be excellent solvents for CO2 capture and electrophile separation, however, some disadvantages, e.g., the relatively high viscosities, limit their eventual large-scale applications. To understand the influences of amino addition on their properties and promote their applications, the microstructure and interionic interaction in two selected amino-functionalized imidazolium ILs, 1-aminoethyl-3-methylimidazolium hexafluorophosphate and 1-aminopropyl-3-butylimidazolium tetrafluoroborate, are studied both for bulk liquid by using molecular dynamics simulations and for isolated ion pair by using ab initio calculations. It is found that the amino addition does not remarkably affect the organization of anions around C2-site, C4-site, and C5-site on imidazolium ring, while it participates in the cation-anion interaction as a new strong site where anions strongly organize around -NH2 and form strong ion-type hydrogen bonds. The condensed phase simulations indicate that their ionic self-diffusion coefficients are on the order of 10(-13) m(2) s(-1) at room temperature, roughly 2 order of magnitude lower than that of conventional imidazolium ILs without -NH2; the isolated ion pair calculations show that such terminal amino-associated interaction reduces the flexibility of alkyl side chains and increases the cation-anion interaction; and these results are qualitatively consistent with their higher experimental viscosities. (c) 2007 American Institute of Chemical Engineers AIChE J, 53: 3210-3221, 2007.