As part of ongoing research focusing on volumetric, acoustic and optical properties of non-aqueous binary liquid mixtures including ionic liquids, here we report, densities, rho, speeds of sound, u and refractive indices, n(D) of room temperature ionic liquids (RTIL) and their binary mixtures with tetrahydrofuran (THF) over the entire composition regime within the temperature range of (293.15 to 323.15) K at 5 K intervals at atmospheric pressure. The ILs studied here were, 1-butyl-3-methylimidazolium hexafluorophosphate ([C(4)mim][PF6]), 1-hexyl-3-methyl imidazolium hexafluorophosphate ([C(6)mim][PF6]) and 1-octyl-3-methylimidazolium hexafluorophosphate ([C(8)mim][PF6]). The excess molar volumes, V-m(E) and excess molar isentropic compressibilities, K-S,m(E) are negative whereas refractive index deviations, Delta(phi)n(D) are positive over the entire mole fraction range and at all temperatures studied. The excess properties were fitted to the Redlich-Kister polynomial equation by the method of least squares with all points weighed equally to derive the binary coefficients and to estimate the standard deviation. The effect of alkyl chain length of ionic liquids on the excess properties has been discussed in terms of interstitial accommodation and ion-dipole interaction between the components of the mixtures. Refractive indices of the mixtures were predicted through several mixing rules and compared with the experimental values by means of the standard deviation (SD) which are in excellent agreement. The Prigogine-Flory-Patterson (PFP) theory has been applied to interpret the V-m(E) results. (C) 2015 Elsevier Ltd. All rights reserved.