© 2019 Infinite dilution activity coefficients (IDAC) of solutes (benzene derivatives, primary alcohols, furan derivatives) in pure 1-butyl-3-methylimidazolium chloride ([Bmim]Cl), in pure 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Bmim][Tf 2 N]) and in an equimolar ionic liquid mixture ([Bmim]Cl and [Bmim][Tf 2 N]) were measured with a chromatographic technique at atmospheric pressure and at (313 to 353) K. The IDAC values for solutes in the ionic liquid mixture were found to lie between their values in the pure ionic liquids meaning that the solute selectivities can be adjusted with ionic liquid composition and that the solute selectivities always lie between those in the pure ionic liquids. IDAC data could be correlated with the ε*-modified Sanchez-Lacombe equation of state with average relative deviations of less than 1.3%. Infinite dilution partition coefficients for solutes in supercritical CO 2 – ionic liquid biphasic systems estimated from interaction parameters obtained from atmospheric pressure IDAC values were found to improve the reliability of high-pressure calculations.