The fundamental natures of the interaction between thiophene and ionic liquids of 1-n-butyl-3-methylimidazolium hexafluorophosphate ([BMIM](+)[PF6](-)) and 1-n-butyl-3-methylimidazolium tetrafluoroborate ([BMIM](+)[BF4](-)) were investigated using ab initio calculations and correlated with previous experimental results. The optimized structures show that the anions of the ionic liquids are situated outside the ring plane of the thiophene, with the fluorine atoms interacting with the hydrogen atoms of the thiophene, and the cation of the ionic liquids approaches the thiophene with its positively charged atoms approaching the negatively charged atoms of TS. It is concluded that thiophene molecules interact with the ionic liquids mainly via Coulombian attraction. Further analysis explained the results obtained from an absorption experiment that the molar ratios of the absorbed thiophene to the ionic liquids were approximately 3.5/1 and 2.4/1 for [BMIM](+)[PF6](-) and [BMIM](+)[BF4](-), respectively. The strong electron donation of the phosphorus atom to the fluorine atoms in the PF6 cluster is believed to be the major factor resulting in the higher molar ratio of thiophene/[BMIM](+)[PF6](-). The other factor is the difference of the compactness between the cation and the anion in the two ionic liquids. (C) 2008 Elsevier B.V. All rights reserved.