Infinite dilution activity coefficient is an important thermodynamic property, which is widely used in evaluating the intermolecular interaction and screening potential solvents for the separation of a fluid mixture by extraction, distillation or selective absorption. In this paper, UNIFAC model and a new group segmentation method are used to correlate infinite dilution activity coefficients based on the experimental data. Among the groups considered in this work, 1,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide ([MMIM][BTI]) and 1,3-dimethylimidazolium dimethyl phosphate ([MMIM][DMP]) are treated as the main groups. A number of infinite dilution activity coefficients of some molecular solutes are well correlated based on the method proposed, which include alkanes, alkenes, aromatics, alcohols, ketones and water in six ionic liquids (ILs), viz. 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imides with alkyl being methyl, ethyl, butyl, hexyl, octyl ([MMIM][BTI], [EMIM][BTI], [BMIM][BTI], [HMIM][BTI], [OMIM][BTI]), and [MMIM][DMP] in the temperature range of 303.15-373.15 K. It is found that the overall accuracy is within 9% in terms of root- mean- square deviation (RMSD); and the overall RMSD is less than 4.2% when the resulting parameters from obtained UNIFAC model are used to predict the vapor pressure of binary mixtures containing IL at varying composition and temperature. The results manifest the applicability of the UNIFAC model and the rationality of the group interaction parameters. (c) 2008 Elsevier B.V. All rights reserved.