The experimental densities (rho), speeds of sound (u), and refractive indices (n(D)) of binary mixtures of 1-octyl-3-methylimidazolium tetrafluoroborate ([C(8)mim][BF4]) with isomers of butanol (1-butanol, 2-methyl-1-propanol, and 2-methyl-2-propanol) were measured at 0.1 MPa and at 298.15, 308.15 and 318.15 K. Excess molar volumes (V-m(E)), excess molar isentropic compressibilities (K-s,m(E)), and deviations in refractive index (Delta(phi)n(D)) were calculated from the experimental data and were fitted to the Redlich-Kister polynomial equation. The results have been interpreted in terms of interstitial accommodation, ion-dipole interactions, formation of the hydrogen bonds, and structural factors involved in the mixture of ionic liquid and molecular organic solvents. Various mixing rules were used to predict the refractive indices and the data have been compared with the experimental results. In addition, analysis of the present V-m(E) data were done through the Prigogine-Flory-Patterson (PFP) theory and the extended real associated solution (ERAS) model. (C) 2016 Elsevier B.V. All rights reserved.