Viscosity (eta) and molar conductivity (Lambda) of aqueous solutions of sodium polystyrene sulfonate (NaPSS) in the presence of some 4-methylpyridinium based ionic liquids (1-Butyl-4-methylpyridinium bromide [BMPyr]Br, 1-Hexyl-4-methylpyridinium bromide [HMPyr]Br and 1-Octyl-4-methylpyridinium bromide [OMPyr]Br) were investigated at T = (288.15, 298.15 and 308.15) K. The viscosity values were correlated with the Wolf equation to obtain the intrinsic viscosity ([eta]) of NaPSS. The intrinsic viscosity of NaPSS decreases by increasing the concentration of ionic liquids due to screens effects of ionic liquids and decreased with increasing alkyl chain length of ILs. The scaling theory is used for the description of electrical conductance of polyelectrolytes. The values of fractions of uncondensed counterions f have been estimated. The effects of the polyelectrolyte concentration, the relative permittivity of the medium, different alkyl chain length and the temperature on the fractions of uncondensed counterions of NaPSS aqueous solutions have been discussed. UV-Vis spectroscopy and Quantum chemical calculations studies are also used to confirm the existence of interactions between NaPSS and ILs. The UV-Vis spectra of NaPSS show that the conformation of NaPSS changes in the presence of ionic liquids; therefore the value of electric multipole moment decreases. Consequently, the absorption intensity of NaPSS was decreased with the addition of ionic liquids. Density functional theory (DFT) has been used to investigate the interactions between [PSS] anion and [RMPyr](+) cations. The molecular electrostatic potential, natural bond orbital methodologies and atoms in molecules are used to analyses the nature of interactions of the ion-pairs [RMPyr](+)[PSS]. (C) 2018 Elsevier Ltd.