For the measurement of limiting activity coefficients different techniques are available. In this work the activity coefficients at infinite dilution gamma(similar to)(i) of alkanes and alkenes in the ionic liquids 1-butyl-3-methylimidazolium tetrafluroborate [BMIM](+)[BF4](-) and 1-hexyl-3-methylimidazolium tetrafluoroborate [HMIM]+[BF4](-) frorn 303.15 K to 353.15 K were measured with the help of the dilutor technique. The new data were compared with already published experimental data measured by using gas-liquid chromatography. Furthermore, the selectivities S-ij(similar to) = gamma(similar to)(i)/gamma(similar to)(j) at infinite dilution for the separation problems hexane-1-hexene and octane-1-octene were determined from the experimental gamma(similar to)(i) data in the investigated ionic liquids and compared with the values obtained for other ionic liquids and conventional selective solvents. (C) 2009 Elsevier B.V. All rights reserved.