Densities (rho) and viscosities (eta) of binary mixtures containing the Protic Ionic Liquid (PIL), pyrrolidinium octanoate with five molecular solvents: water, methanol, ethanol, n-butanol, and acetonitrile are determined at the atmospheric pressure as a function of the temperature and within the whole composition range. The refractive index of all mixtures (n(D)) is measured at 298.15 K. The excess molar volumes V-E and deviation from additivity rules of viscosities eta(E) and refractive index Delta(phi)n, of pyrrolidinium octanoate solutions were then deduced from the experimental results as well as apparent molar volumes V-phi i, partial molar volumes (V) over bar (m,i) and thermal expansion coefficients alpha(p). The excess molar volumes V-E are negative over the entire mole fraction range for mixture with water, acetonitrile, and methanol indicating strong hydrogen-bonding interaction for the entire mole fraction. In the case of longest carbon chain alcohols (such as ethanol and n-butanol) + pyrrolidinium octanoate solutions, the V-E variation as a function of the composition describes an S shape. The deviation from additivity rules of viscosities is negative over the entire composition range for the acetonitrile, methanol, ethanol, and butanol, and becomes less negative with increasing temperature. Whereas, eta(E) of the {[Pyrr][C7CO2] + water) binary mixtures is positive in the whole mole fraction range and decreases with increasing temperature. the excess Gibbs free energies of activation of viscous flow (Delta G*(E)) for these systems were calculated. The deviation from additivity rules of refractive index Delta(phi)n are positive over the whole composition range and approach a maximum of 0.25 in PIL mole fraction for all systems. The magnitude of deviation for Delta(phi)n describes the following order: water > methanol > acetonitrile > ethanol. Results have been discussed in terms of molecular interactions and molecular structures in these binary mixtures. (c) 2010 Elsevier Ltd. All rights reserved.