Excess molar volume V-E, speed of sound u, isentropic compressibility kappa(s) and excess isentropic compressibility kappa(E)(s) data for binary S mixtures of 1-n-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]) or 1-n-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]) in methanol and acetonitrile as function of composition under atmospheric pressure at T = (298.15 to 318.15) K have been used to test applicability of Prigogine-Flory-Patterson (PFP) theory. In the case of [BMIM][PF6] in methanol and acetonitrile mixtures, data have been reported previously. Interaction parameters have been calculated from V-E values for all studied systems. The PFP theory is capable of reproducing the main features of the experimental V-E, u, kappa(s), and kappa(E) values by using a fitted binary interaction parameter. An analysis of each of three contributions viz. interactional, free volume and P* to V-E at equimolar concentration was applied for investigation of solute-solvent interaction and structural features of each components in the studied mixtures. (c) 2006 Elsevier Ltd. All rights reserved.