Activity coefficients at infinite dilution (gamma(infinity)), vapor-liquid equilibria (VLE) and excess enthalpies (H-E) were measured for alkanes and alkenes in the ionic liquids 1-butyl-1-methyl-pyrrolidinium bis(trifluoromethylsulfonyl)imide [BMPYR](+)[BTI](-), 1-hexyl-1-methyl-pyrrolidinium bis(trifluoromethylsulfonyl)imide [HMPYR](+)[BTI](-) and 1-octyl-1-methyl-pyrrolidinium bis(trifluorornethylsulfonyl)imide [OMPYR](+)[BTI](-). The new data and the already available data for systems of ionic liquids consisting of the cation pyrrolidinium and the anion bis(trifluoromethylsulfonyl)imide with alkanes and alkenes were used to fit the group interaction parameters for the modified UNIFAC (Dortmund) model. A comparison of the experimental and predicted results shows that modified UNIFAC (Dortmund) is able to predict reliably the excess enthalpies and the phase equilibrium behavior of these systems. With the new parameters fitted for modified UNIFAC, the range of applicability of software tools for the development of separation processes was extended. (c) 2008 Elsevier B.V. All rights reserved.