In this work, the experimental values of density, speed of sound, refractive index and dynamic viscosity have been obtained from T = (293.15 to 343.15) K for the three phosphonium-based ionic liquids: tributyl methyl phoshponium methylsulfate (P-4441 C1SO4), tributyl ethyl phosphonium diethylphosphate (P-4442 (C-2)(2)PO4) and tributyl octyl phosphonium chloride (P-4448 Cl). The isentropic compressibility has been calculated by means of the Laplace equation from the experimental speed of sound results for the three ionic liquids at different temperatures. Density, speed of sound, refractive index and isentropic compressibility have been correlated by polynomial equations. The Lorentz-Lorenz, Dale-Gladstone, Eykman, Oster, Arago-Biot, Newton and modified Eykman equations were the empirical models used to correlate satisfactorily the relationship between the densities and refractive indices of the ionic liquids selected. The temperature dependence of the experimental dynamic viscosities for the ionic liquids selected can be described by an Arrhenius-like law and by VFT equations. The Riedel, Narsimham, Bradford-Thodos, Yen-Woods, Rackett, Spencer-Danner, Gunn-Yamada, Hankinson-Thomson (COSTALD model), VSY, VSD, MH and LGM equations were employed to predict the densities of the pure ionic liquids. (c) 2013 Published by Elsevier Ltd.