Density and speed of sound measurements have been made on the systems containing the ionic liquid 1-n-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]) and some organic solvents having a wide range of dielectric constants. Similar studies have been carried out for tetrabutylammonium hexafluorophosphate ([TBA][PF6]), which has common anion ([PF6](-)) with the studied ionic liquid. For the systems investigated., the apparent molar volumes and apparent molar isentropic compressibilities were determined and fitted to the Redlich-Mayer and the Pitzer equations from which the corresponding limiting values were obtained. These limiting values were used to obtain some information about ion-solvent interactions. Furthermore, using the ionic limiting apparent volume values for [TBA](+) from the literature and limiting apparent molar volume for the ionic liquid and [TBA][PF6] obtained in this work, the ionic limiting apparent molar volume values for the cation [BMIM](+) in different organic solvents were also estimated. (C) 2005 Elsevier Ltd. All rights reserved.