Electrolytic conductivities (Lambda), densities (rho), viscosities (eta), and ultrasonic speed (u) of an ionic liquid (IL) tetrabutylphosphonium tetrafluoroborate [Bu4PBF4] have been studied in dimethylsulfoxide (DMSO), dimethylacetamide (DMA), and dimethylformamide (DMF) at T = 298.15 K. The limiting molar conductivity (Lambda(0)), the association constant (K-A), and the distance of closest approach of ions (R) have been evaluated using the Fuoss conductance equation (1978). The molar conductivities observed were explained by the formation of ion-pairs (M+ + X <-> MX). The Walden product is obtained and discussed. Ion-solvent interactions have been interpreted in terms of apparent molar volumes (phi(V)) and viscosity B-coefficients which are obtained from the results supplemented with densities and viscosities, respectively. The limiting apparent molar volumes (phi(0)(V)), experimental slopes (S*(V)) derived from the Masson equation and viscosity A and B coefficients using the Jones-Dole equation have been interpreted in terms of ion-ion and ion-solvent interactions respectively. The adiabatic compressibility (beta) has been evaluated using the ultrasonic speed (u) values. Finally values of the limiting partial molar adiabatic compressibility (phi(0)(K)) are calculated and discussed. (C) 2012 Elsevier Ltd. All rights reserved.