Infinite dilution activity coefficients y(i)(infinity) have been measured for various solutes (alkanes. alkenes. cycloalkanes, cycloalkenes. aromatics. alcohols, ketones, esters, ethers, and water) in the five ionic liquids 1-methyl-3-methyl-imidazolium methylsulfate [MMIM](+)[CH3SO4](-). 1-methyl-3-methyl-imidazolium methoxyethylsulfate [MMIM](+)[CH3OC2H4SO4](-). 1-methyl-3-methyl-imidazolium dimethylphosphate [MMIM](+)[(CH3)=PO4](-), N-ethylpyridinium bis(trifluoromethylsulfonyl) imide [C5H5NC2H5](+)[(CF3SO2)(2)N](-), and pyridiniumethoxyethylsulfate [C5H5NH](+)[C2H5OC2H4OSO3](-) in the temperature range from 303.15 to 373.15 K. Additionally. densities of the investigated ionic liquids were measured in the temperature range from 293.15 to 353.15 K. Using all available gamma(i)(infinity)-data, the selectivities S-ij(infinity)= gamma(i)(infinity)/gamma(i)(infinity) and the capacities k(i)(infinity) = 1/gamma(i)(infinity) at infinite dilution were determined for different separation problems not only for ionic liquids, but also N-methyl-2-pyrrolidone (NMP) and ethylene glycol. (C) 2004 Elsevier B.V All rights reserved.